Add file-based registry provider to thv-registry-api

[jhrozek]

Extends the registry API server to support reading registry data from local files in addition to Kubernetes ConfigMaps. This enables deployment scenarios where ConfigMaps are mounted as volumes rather than accessed via the Kubernetes API.

The server now accepts either:

• --configmap flag: Reads from Kubernetes ConfigMap via API
• --file flag: Reads from mounted file (for volume-mounted ConfigMaps)

[codecov]

Codecov Report

❌ Patch coverage is 45.71429% with 114 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.06%. Comparing base (24a2f37) to head (4a58efa).
⚠️ Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/thv-registry-api/app/serve.go	0.00%	72 Missing ⚠️
...-registry-api/internal/service/provider_factory.go	77.50%	12 Missing and 6 partials ⚠️
...pi/internal/service/mocks/mock_provider_factory.go	0.00%	15 Missing ⚠️
...gistry-api/internal/service/mocks/mock_provider.go	0.00%	8 Missing ⚠️
...thv-registry-api/internal/service/file_provider.go	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1909      +/-   ##
==========================================
+ Coverage   46.90%   47.06%   +0.15%     
==========================================
  Files         220      223       +3     
  Lines       27319    27522     +203     
==========================================
+ Hits        12814    12953     +139     
- Misses      13530    13584      +54     
- Partials      975      985      +10     

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[coveralls]

coverage: 44.419% (+0.2%) from 44.242%
when pulling 4a58efa on thv-registry-api-file
into 24a2f37 on main.

[dmartinol]

/lgtm apart from a minor comment

[jhrozek]

there was one lint issue, the tests failed only because coveralls timed out