authn: Add NewConfigKeychain to load a config from explicit path

[imjasonh]

Fixes #1595

cc @hasheddan

[jonjohnsonjr]

Want to rebase this given the refresher stuff?

[codecov-commenter]

Codecov Report

Merging #1603 (e72603c) into main (9aa45a1) will decrease coverage by 0.01%.
The diff coverage is 90.47%.

❗ Current head e72603c differs from pull request most recent head 677d403. Consider uploading reports for the commit 677d403 to get more accurate results

@@            Coverage Diff             @@
##             main    #1603      +/-   ##
==========================================
- Coverage   72.08%   72.08%   -0.01%     
==========================================
  Files         120      120              
  Lines        9782     9814      +32     
==========================================
+ Hits         7051     7074      +23     
- Misses       2011     2017       +6     
- Partials      720      723       +3     

Impacted Files	Coverage Δ
pkg/authn/keychain.go	89.05% <87.23%> (-5.45%)	⬇️
pkg/v1/google/keychain.go	87.87% <100.00%> (+0.37%)	⬆️
pkg/v1/google/list.go	71.27% <100.00%> (+0.46%)	⬆️

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