Reduce auth package surface

[jhrozek]

The auth package had too many symbols exported. Let's export only what's needed and make most of the symbols internal to make the intent of how the package is to be used more explicit.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.41935% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.58%. Comparing base (b78ee5a) to head (665ae2b).

Files with missing lines	Patch %	Lines
internal/auth/mocks/mock_validator.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #196   +/-   ##
=======================================
  Coverage   66.58%   66.58%           
=======================================
  Files          60       60           
  Lines        3765     3765           
=======================================
  Hits         2507     2507           
  Misses       1101     1101           
  Partials      157      157           

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