feat: add UNKNOWN enum value to Vulnerability sources

apiserver/src/main/java/org/dependencytrack/model/Vulnerability.java

@@ -130,7 +130,8 @@ public enum Source {
         RETIREJS, // Retire.js
         INTERNAL,  // Internally-managed (and manually entered) vulnerability
         OSV, // Google OSV Advisories
-        SNYK;    // Snyk Purl Vulnerability
+        SNYK,    // Snyk Purl Vulnerability
+        UNKNOWN; // Unknown or unrecognized vulnerability source
 
         public static boolean isKnownSource(String source) {
             return Arrays.stream(values()).anyMatch(enumSource -> enumSource.name().equalsIgnoreCase(source));