[feat] support custom GitHub OAuth2 auth and token URLs

[analytically]

Feature

Which issue does this PR fix:

Support custom Github OAuth2 auth and token URLs

[andaaron]

Thank you for your PR, can you also add an example configuration under https://github.com/project-zot/zot/tree/main/examples?

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 90.95%. Comparing base (c87f489) to head (fd87859).

Files with missing lines	Patch %	Lines
pkg/api/authn.go	50.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3024      +/-   ##
==========================================
- Coverage   90.96%   90.95%   -0.02%     
==========================================
  Files         176      176              
  Lines       32940    32951      +11     
==========================================
+ Hits        29965    29970       +5     
- Misses       2245     2249       +4     
- Partials      730      732       +2     

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[andaaron]

@analytically can you also add a test to verify the new configuration?

[analytically]

@andaaron a test that runs against GitHub Enterprise? Or just config object validation?

[andaaron]

I was thinking maybe we could mock it, similar to what we have in

zot/pkg/test/auth/oidc.go

Line 13 in c87f489

func MockOIDCRun() (*mockoidc.MockOIDC, error) {

[rchincha]

Support custom Github OAuth2 auth and token URLs

^ could you also point to pointers/docs about why this PR is needed.

@analytically thanks for taking the time for this PR.