Add mTLS support

[ukaj]

Issue:

• Currently PyGitHub doesn't allow to authenticate using client certs (mTLS), because cert property is not passed to requests call

Example:

gh = Github(cert='/tmp/client_cert.pem')
TypeError: Github.__init__() got an unexpected keyword argument 'cert'

Proposed fix:

• Add cert property to Github object, which will allow to use cert for requests as described in documentation https://requests.readthedocs.io/en/latest/user/advanced/#client-side-certificates

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e584a90) 96.74% compared to head (d7496fe) 96.72%.
Report is 34 commits behind head on main.

Files	Patch %	Lines
github/Auth.py	75.00%	1 Missing ⚠️
github/Requester.py	75.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2811      +/-   ##
==========================================
- Coverage   96.74%   96.72%   -0.02%     
==========================================
  Files         139      139              
  Lines       14184    14189       +5     
==========================================
+ Hits        13722    13725       +3     
- Misses        462      464       +2     

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[EnricoMi]

Does GH or GH Enterprise support mTLS authentication? Can this be combined with any other available authentication like token or login?

[EnricoMi]

@ukaj ping

[EnricoMi]

Once we have access to the session of the Requester (#2865), this can be implemented as an auth.Auth.