fix(client/rs): do not error when issuer discovery has no introspection endpoint

[hhromic]

This PR fixes client/rs to not error when an issuer discovery returns no introspection endpoint.

Summary of changes:

• newResourceServer() will only fill rs.tokenURL and rs.introspectURL if they were not given as static endpoints using WithStaticEndpoints(). This allows the discovery mechanism to always respect the user-provided static endpoints and only fill them when one or the other was not statically provided. Also, given that config.introspectURL is optional in the OIDC Discovery spec, it is no longer enforced to be present after discovery.
• Introspect() will now fail if the passed ResourceServer does not have an IntrospectionURL() set. This allows for graceful errors in case issuers do not provide an introspection endpoint.
• Added unit tests covering all situations using discovery and static endpoints.

Fixes #413

Definition of Ready

• I am happy with the code
• Short description of the feature/issue is added in the pr description
• PR is linked to the corresponding user story
• Acceptance criteria are met
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• Deviations from the acceptance criteria and design are agreed with the PO and documented.
• No debug or dead code
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• Where possible E2E tests are implemented
• Documentation/examples are up-to-date
• All non-functional requirements are met
• Functionality of the acceptance criteria is checked manually on the dev system.

[muhlemmer]

Thanks for this excellent contribution and tests!

[codecov]

Codecov Report

Merging #414 (974bdcb) into main (9e62498) will increase coverage by 0.40%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##             main     #414      +/-   ##
==========================================
+ Coverage   52.28%   52.69%   +0.40%     
==========================================
  Files          74       74              
  Lines        5711     5718       +7     
==========================================
+ Hits         2986     3013      +27     
+ Misses       2461     2445      -16     
+ Partials      264      260       -4     

Impacted Files	Coverage Δ
pkg/client/rs/resource_server.go	53.08% <81.81%> (+23.35%)	⬆️

... and 2 files with indirect coverage changes

[github-actions]

🎉 This PR is included in version 2.6.4 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[muhlemmer]

@hhromic if you are interested, you can get some contributor swag through this google form: https://docs.google.com/forms/d/e/1FAIpQLSc-8t4G7HHCnxLKdGR7F3vLtKu31f7K4fdlqW4j0cirMYPUEA/viewform

[hhromic]

@muhlemmer thanks for the kind offer and very nice to see your company supports open source in such a great way!
For the moment I'm good, this was a very small contribution hehe. Perhaps next time! 🍻