[mdatagen] Replace attributes optional field with requirement_level

[dmitryax]

metadata.yaml schema change for attributes: boolean optional field replaced with enum requirement_level field with the following options:
- required: attribute is always included and cannot be excluded
- conditionally_required: attribute is included by default when certain conditions are met (replaces optional: true)
- recommended: attribute is included by default but can be disabled via configuration (replaces optional: false)
- opt_in: attribute is not included unless explicitly enabled in user config

The recommended option is assumed when requirement_level isn't specified explicitly which is the case for the vast majority use cases.

metadata.yaml files upgrade guideline: Replace optional: true with requirement_level: conditionally_required and optional: false with requirement_level: recommended (or omit for default) in metadata files.

This change is needed for #13900

[codecov]

Codecov Report

❌ Patch coverage is 91.07143% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.59%. Comparing base (8b09905) to head (e6d2492).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/metadata.go	83.33%	5 Missing ⚠️

❌ Your patch status has failed because the patch coverage (91.07%) 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   #13913      +/-   ##
==========================================
- Coverage   91.61%   91.59%   -0.03%     
==========================================
  Files         655      655              
  Lines       42782    42814      +32     
==========================================
+ Hits        39195    39215      +20     
- Misses       2764     2774      +10     
- Partials      823      825       +2     

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[dmitryax]

Merging given that I've got one approval and addressed the comments