feat: add Siemens REST API linter

api-linter/.npmignore

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+/.npm-cache
+/test
+/example

api-linter/README.md

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+# Siemens API Linter
+
+This project implements a linter reporter for the [Stoplight Spectral](https://docs.stoplight.io/docs/spectral/674b27b261c3c-overview) for API specifications.
+The ruleset has been created based on the [Siemens Xcelerator API guidelines 2.5.0](https://developer.siemens.com/guidelines/api-guidelines/rest/index.html).
+
+## Getting started
+### Directory structure
+
+- `example/`: contains a sample API spec file
+- `rulesets/`: contains the ruleset files those follow the Siemens Xcelerator API guidelines
+- `src/`: contains the source files for creating the linter report
+- `test/`: jest test cases for different rules in the rulesets
+
+### Installation
+
+In order to include the rulesets and provide linting within your project, you can install the package from this project with this `@siemens/api-linter` package. 
+
+```json
+{
+  "scripts": {
+      "test": "api-linter -s reference-api-specs/openapi.yml -r reference-api-specs/.spectral.yml"
+  },
+  "devDependencies": {
+      "@siemens/api-linter": "^0.8.4"
+  }
+}
+```
+
+The `@siemens/api-linter` package includes the new Siemens REST API linter tool with html report capability.
+
+In order to trigger the linter on your API specifications, it provides a script to execute the linter within the `package.json` specification.
+
+```json
+  "scripts": {
+      "test": "api-linter -s reference-api-specs/openapi.yml -r reference-api-specs/.spectral.yml"
+  }
+```
+Developers can change the spec file path with `-s` option, ruleset file path with `-r` option accordingly in the scripts.<br>
+Or just remove them and set them in the command line as:<br>
+
+Script:
+```json
+  "scripts": {
+      "test": "api-linter"
+  }
+```
+Command:
+```groovy
+npm test -s reference-api-specs/openapi.yml -r reference-api-specs/.spectral.yml
+```
+
+For more options:
+
+```
+Usage: api-linter -s "path-to-spec-file" -r "path-to-rule-file" [-f "fail-severity"] [-c "console-severity"] [-v "api-versioning"] [-a "api-security"]
+
+Options:
+  -s, --specPath <openapi spec>        path to openapi specification
+  -r, --rulesetPath <rule file>        path to rules file
+  -f, --failSeverity <fail severity>   test fails when met result the severity equal or higher than it (choices: "error", "warn", "info", "hint", default: warn)
+  -c, --consoleSeverity <console severity> console output message for linter result no matter job failed or succeeded (choices: "error", "warn", "info", "hint", default: warn)
+  -o, --outputFilename <output filename> specify the output filename with or without .html extension (default: linter-result.html)
+  -p, --jsonFile <output json file>, output json file with origin spectral result data
+  -v, --apiVersioning <api versioning> the api versioning way (choices: "ignore", "url", "header", default: ignore)
+  -a, --apiSecurity <authorization security> if enable authorization security (choices: "n/f/no/false/y/t/yes/true" default: n)
+  --resolve enables follow external $refs (same as Spectral --resolve) (default is not enabled, i.e. external references are not resolved and ignored)
+  ```
+
+A linter report will be generated according to the execution as:
+
+![Example Image](linter-report.png)
+
+### Linting within CI/CD
+
+You can trigger linting on your CI/CD pipeline executing `npm test` within the `.gitlab-ci.yml` CI configuration file.
+
+See the following configuration example:
+
+```yaml
+build:
+  script:
+    - npm test
+  artifacts:
+    name: "api-linter-report"
+    paths:
+      - linter-result.html
+    when: always
+```
+
+> The Pipeline will fail if linter results with severity of `warn` (By default) or higher found!
+
+> You may download or browse the report file in the job artifact page directly no matter CI job failed or succeeded.
+
+### Selection of rulesets
+
+The package provides several rulesets according to Siemens REST API guidelines.<br>
+> They can be included as needed by providing a project specific `.spectral.yml` file on the projects root directory as follows:
+```yaml
+extends:
+  - "@siemens/api-linter/rulesets/siemens-api-media-type.yml"
+  - "@siemens/api-linter/rulesets/siemens-api-versioning.yml"
+  - "@siemens/api-linter/rulesets/siemens-api-error-reporting.yml"
+  - "@siemens/api-linter/rulesets/siemens-api-filtering.yml"
+  - "@siemens/api-linter/rulesets/siemens-api-sparse-fieldsets.yml"
+  - "@siemens/api-linter/rulesets/siemens-api-pagination.yml"
+  - "@siemens/api-linter/rulesets/siemens-api-sorting.yml"
+  - "@siemens/api-linter/rulesets/siemens-api-common-operation.yml"
+  - "@siemens/api-linter/rulesets/siemens-api-security.yml"
+```
+> You MAY use as below to achieve this for guideline linting `WITHOUT` `"spectral:oas"`.
+```yaml
+extends:
+  - "@siemens/api-linter/rulesets/siemens-api-express.yml"
+```
+> `OR` just extends the `ALL-IN-ONE` ruleset which with `"spectral:oas"` enabled.
+```yaml
+extends:
+  - "@siemens/api-linter/rulesets/siemens-api.yml"
+```
+### Disable any Ruleset or Rules
+> You can disable any rules/rulesets as you wish by [Stoplight Spectral](https://docs.stoplight.io/docs/spectral/674b27b261c3c-overview).
+```yaml
+#e.g. disable spectral:oas ruleset and disable Siemens-API-[400] rules from the extended ruleset
+extends:
+  - "@siemens/api-linter/rulesets/siemens-api.yml"
+  - ["spectral:oas", "off"]
+
+rules:
+  Siemens-API-[400]: false
+  Siemens-API-[401]: false
+```
+
+### Define rule dependency relations
+> Sometimes, if you don't want to follow a ruleC that under a high level ruleP, which means when ruleP failed, you don't want to check ruleC.
+Then we can define the rules with `"x-dependsOn"` attribute.
+```yaml
+#e.g. Siemens-API-[101.8.1] depends on Siemens-API-[101.8]
+extends:
+  - "@siemens/api-linter/rulesets/siemens-api.yml"
+
+rules:
+  Siemens-API-[101.8.1]: 
+    x-dependsOn:
+      - Siemens-API-[101.8]
+```
+
+## Integration Using JavaScript
+> In case you want to handle the linting results by yourself
+```javascript
+const {validator} = require('@siemens/api-linter/src/extension');
+const {DiagnosticSeverity} = require('@stoplight/types');
+const path = require("path");
+const validate = validator();
+
+const validateDoc = function(){
+    const ruleSetFilePath = path.resolve(__dirname, 'reference-api-specs/.spectral.yml');
+    const apiSpecFilePath = path.resolve(__dirname, 'reference-api-specs/openapi-prod.yml');
+    validate(apiSpecFilePath, ruleSetFilePath).then(result => {
+        const diagnostics = [];
+        for (let msg of result.results || []){
+            if (msg.severity <= DiagnosticSeverity.Information ){
+                var diagnostic = { value: msg.code, severity: msg.severity, message: msg.message };
+                diagnostics.push(diagnostic);
+            }
+        }
+        // Add your own processes to handle diagnostic array;
+    });
+}
+validateDoc();
+```
+