UpstreamRegistry type, converters and test code

[dmartinol]

Related to stacklok/toolhive-registry-server#76
Adding UpstreamRegistry to model the aggregated upstream registry model
Will be used by stacklok/toolhive-registry-server#104

[codecov]

Codecov Report

❌ Patch coverage is 57.62712% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.76%. Comparing base (cead1c3) to head (3267a2d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/registry/types/upstream_registry.go	0.00%	13 Missing ⚠️
pkg/registry/converters/registry_converters.go	73.91%	6 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2567      +/-   ##
==========================================
- Coverage   55.95%   55.76%   -0.20%     
==========================================
  Files         316      315       -1     
  Lines       29766    29664     -102     
==========================================
- Hits        16655    16541     -114     
- Misses      11652    11662      +10     
- Partials     1459     1461       +2     

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