Fix Issue #576 collaborator by username

[0xacx]

Description

Fixes #576
Adds a method to check if GHUser is a collaborator on that repository. You can find the github documentation for that endpoint here.

Before submitting a PR:

• Changes must not break binary backwards compatibility. If you are unclear on how to make the change you think is needed while maintaining backward compatibility, CONTRIBUTING.md for details.
• Add JavaDocs and other comments as appropriate. Consider including links in comments to relevant documentation on https://docs.github.com/en/rest .
• Add tests that cover any added or changed code. This generally requires capturing snapshot test data. See CONTRIBUTING.md for details.
• Run mvn -D enable-ci clean install site locally. If this command doesn't succeed, your change will not pass CI.
• Push your changes to a branch other than main. You will create your PR from that branch.

When creating a PR:

• Fill in the "Description" above with clear summary of the changes. This includes:
  • If this PR fixes one or more issues, include "Fixes #" lines for each issue.
  • Provide links to relevant documentation on https://docs.github.com/en/rest where possible.
• All lines of new code should be covered by tests as reported by code coverage. Any lines that are not covered must have PR comments explaining why they cannot be covered. For example, "Reaching this particular exception is hard and is not a particular common scenario."
• Enable "Allow edits from maintainers".

[0xacx]

@bitwiseman Could I please have access to the test org so I can record the response for the wiremock stub?
Thank you!

[bitwiseman]

@0xacx Invitation sent.

[0xacx]

@bitwiseman Unfortunately I haven't logged in for a long while and the invitation expired. Any chance you could resend it?
Thanks!

[bitwiseman]

@0xacx Sent invite again.

[codecov]

Codecov Report

Merging #1431 (be1aaa5) into main (262cf84) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##               main    #1431      +/-   ##
============================================
- Coverage     78.81%   78.80%   -0.01%     
- Complexity     2114     2115       +1     
============================================
  Files           202      202              
  Lines          6429     6432       +3     
  Branches        361      362       +1     
============================================
+ Hits           5067     5069       +2     
  Misses         1152     1152              
- Partials        210      211       +1     

Impacted Files	Coverage Δ
src/main/java/org/kohsuke/github/GHRepository.java	68.92% <66.66%> (-0.02%)	⬇️

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[0xacx]

@bitwiseman Could you or any other maintainer please approve running the workflows? Thanks!

[0xacx]

@bitwiseman The Java 8 build is failing at the codecov job, because of an expired certificate. It looks random to me, as if it was indeed expired, all branches should suffer from that. Could we try a rerun or is there something else I should do?

[bitwiseman]

Nothing for you to do right now. I'll get it fixed up and rerun.