feat: support string comparison for jwt-role-claim-key

[taimoorzaeem]

Fixes #1536.

@steve-chavez Please review.

[steve-chavez]

@taimoorzaeem How about some docs for this feature?

I think we should:

• Have a section after https://docs.postgrest.org/en/v12/references/auth.html#jwt-claims-validation called something like "JWT Role Claim Extraction"
• Move the details about JSPath on jwt-role-claim-key there.
• Add the new comparison operators on the new section.
• Finally, put a link on jwt-role-claim-key to this new section.

[steve-chavez]

Codecov still complains about some lines, can you try and clear those?

[taimoorzaeem]

@steve-chavez I have changed the $== operator with ==^. Turns out that the $ character does not work well inside environment variable values which was causing some issues during testing. I could have overlooked this if it wasn't for code-cov not passing enough. Huh, another win for code-cov testing 🚀.

[steve-chavez]

I have changed the $== operator with ==^

@taimoorzaeem Are those functionally equivalent?

[taimoorzaeem]

@taimoorzaeem Are those functionally equivalent?

Yes, absolutely.

[steve-chavez]

Great work! 🚀