Improve registry API behaviour for unreachable registries

[dmjb]

Previously a registry could be added without validating that it exists. When calling a registry API which referred to a non existent registry, it would time out for 10 seconds and return a 500.

Change the behaviour to:

1. Validate that the registry is contactable and matches the contract we expect.
2. Reduce timeout and return 504 when using an unreachable registry.

[codecov]

Codecov Report

❌ Patch coverage is 70.18349% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.97%. Comparing base (e6d3bde) to head (a381b03).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/api/v1/registry.go	66.10%	13 Missing and 7 partials ⚠️
pkg/config/registry_service.go	67.24%	15 Missing and 4 partials ⚠️
pkg/config/registry.go	67.85%	17 Missing and 1 partial ⚠️
pkg/registry/provider_remote.go	79.31%	3 Missing and 3 partials ⚠️
pkg/registry/provider_api.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3463      +/-   ##
==========================================
+ Coverage   64.90%   64.97%   +0.06%     
==========================================
  Files         394      398       +4     
  Lines       38402    38644     +242     
==========================================
+ Hits        24926    25109     +183     
- Misses      11530    11577      +47     
- Partials     1946     1958      +12     

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