fallback to json default cfg path if yaml does not exist

[miyunari]

Closes #1811

Summary

The PR #1720 has changed the default config path from /etc/fulcio-config/config.json to /etc/fulcio-config/config.yaml. This caused problems with non-explicitly configured environments.

Release Note

Check and fallback to default JSON config if the default YAML configuration file did not exist.

Documentation

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 50.19%. Comparing base (cf238ac) to head (53fc3e7).
Report is 212 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/app/serve.go	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1810      +/-   ##
==========================================
- Coverage   57.93%   50.19%   -7.75%     
==========================================
  Files          50       70      +20     
  Lines        3119     4202    +1083     
==========================================
+ Hits         1807     2109     +302     
- Misses       1154     1858     +704     
- Partials      158      235      +77     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[haydentherapper]

Thanks!

[haydentherapper]

Will need to rebase once #1814 merges.

[haydentherapper]

Needs a rebase.

[miyunari]

Needs a rebase.

Sry @haydentherapper, just saw this and rebased it now 👍