[cmd/mdatagen] Add basic support for entities to metadata.yaml schema

[dmitryax]

When entities are defined, mdatagen generates AssociateWith{EntityType}() methods on ResourceBuilder that associate resources with entity types using the entity refs API. The entities section is backward compatible - existing metadata.yaml files without entities continue to work as before. This change is fully additive and conditionally executed only if metadata.yaml has entities defined.

The generated Go API is experimental and will change once #14039 is merged. The generated documentation will also be updated for better readability

Updates #14051

[codecov]

Codecov Report

❌ Patch coverage is 69.76744% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.57%. Comparing base (2646139) to head (9e3a53f).

Files with missing lines	Patch %	Lines
...econnector/internal/metadata/generated_resource.go	0.00%	7 Missing ⚠️
cmd/mdatagen/internal/metadata.go	82.85%	3 Missing and 3 partials ⚠️

❌ Your patch check has failed because the patch coverage (69.76%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14053      +/-   ##
==========================================
- Coverage   91.59%   91.57%   -0.02%     
==========================================
  Files         655      655              
  Lines       42786    42829      +43     
==========================================
+ Hits        39188    39222      +34     
- Misses       2772     2779       +7     
- Partials      826      828       +2     

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[ChrsMark]

Overall looks great. Left a small nit