Simplify redirect URI validation in autheserver upstream package to use fosite

[jhrozek]

As I'm going through the motions of plugging in the upstream proxy providers into the authproxy "frontend", I'm noticing things in the upstream code that could be simplified. This is one of them.

Replace hand-rolled redirect URI validation with fosite functions. This is our own callback URL where upstream IDPs redirect back to us - the upstream IDP validates it against their registered URIs anyway.

This aligns with the DCR package which already uses fosite for the same purpose.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.65%. Comparing base (a5a7d48) to head (dd77937).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/authserver/upstream/oauth2.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3339      +/-   ##
==========================================
- Coverage   64.68%   64.65%   -0.03%     
==========================================
  Files         369      369              
  Lines       35963    35956       -7     
==========================================
- Hits        23264    23249      -15     
- Misses      10868    10877       +9     
+ Partials     1831     1830       -1     

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