feat: Support custom auth schemes for Github API

[soceanainn]

Background

When looking to support custom authentication with Github (by proxying to Github API through a custom authenticated API), the token auth-scheme in the Authorization header may not be appropriate.

For example, when looking to use AWS Lambda function to proxy Github API requests and using AWS Cognito for authentication (Cognito is the user management service offered by AWS out of the box), authenticated requests to the Lambda function must use Bearer scheme.

Note: Github API does seem to accept either Bearer or token, and even appears to prefer and sometimes mandate Bearer according to their API docs:
https://docs.github.com/en/rest/authentication/authenticating-to-the-rest-api?apiVersion=2022-11-28#about-authentication

However - changing this would be a potentially breaking change and I'm hesitant to change something fundamental to the Github backend implementation in this PR. This PR would allow us to test that the normal Github implementation works with the Bearer scheme and change that to the new default in future (if desired) - but that's well outside the scope of this change.

Changes

1. Allow configuring the authScheme used in Authorization headers for Github backend, defaulting to the expected default value of token.
2. Add basic test ensuring that overriding authScheme works for API spec. No equivalent tests exist in implementation spec so I haven't added equivalent tests for that.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.83%. Comparing base (9379d3c) to head (d0d4d2e).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1086   +/-   ##
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  Coverage   55.82%   55.83%           
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  Files         259      259           
  Lines       12360    12362    +2     
  Branches     3108     3110    +2     
=======================================
+ Hits         6900     6902    +2     
  Misses       5048     5048           
  Partials      412      412           

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