Add a function to check if context string is supported

[M-AlNoaimi]

Hi, this PR introduces a new API function:

OQS_API bool OQS_SIG_supports_ctx_str(const char *alg_name);

• This allows for projects like liboqs-python to be able to check whether a given siguature algorithm supports context strings or not, without relying on internal structures.

This was done because I noticed liboqs-python's CI kept failing, due to context string-related tests running on algorithms that don't support them, there was a work-around in the pytest but that became a bit convoluted (at least more than if liboqs itself had a feature like this).

I was able to have a C-side test to check API and struct consistency for all enabled signature algorithms, I can add this in to the PR if needed, I kept it back as I'm not totally sure if there needs to be a specific test-case for this.

./build/tests/test_sig Dilithium2
OQS_SIG_supports_ctx_str API test passed for all enabled algorithms.
Testing signature algorithms using liboqs version 0.13.1-dev
...
Sample computation for signature Dilithium2
verification passes as expected

[dstebila]

Thanks very much! I don't think the consistency test needs to be added, so the PR looks good as is to me.

[SWilson4]

LGTM! Thanks for the contribution.

[SWilson4]

@open-quantum-safe/liboqs-committers This adds an API call, so please do glance at it. @vsoftco, this might be of interest to you in particular given open-quantum-safe/liboqs-python#111.

[christianpaquin]

LGTM

[SWilson4]

Thanks for the contribution!