Add proposal for Authenticating Users via GitHub

[cfryanr]

Rendered proposal text can be viewed here (until this PR is merged, and then it could be viewed here).

[benjaminapetersen]

Quick ref to your POC: session token lifetime & length: #1857

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 38.58%. Comparing base (f0a43e0) to head (2af6f4c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1860      +/-   ##
==========================================
+ Coverage   38.56%   38.58%   +0.01%     
==========================================
  Files         350      350              
  Lines       44514    44514              
==========================================
+ Hits        17168    17174       +6     
+ Misses      26831    26826       -5     
+ Partials      515      514       -1     

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