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 75.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.90%. Comparing base (0f4c563) to head (847be0f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/registry/converters/registry_converters.go	75.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2567      +/-   ##
==========================================
+ Coverage   55.88%   55.90%   +0.02%     
==========================================
  Files         311      312       +1     
  Lines       29488    29512      +24     
==========================================
+ Hits        16478    16498      +20     
- Misses      11571    11572       +1     
- Partials     1439     1442       +3     

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[dmartinol]

removed the unnecessary functions (will keep in the registry server and eventually remove)
@blkt ^