[chore][cmd/mdatagen] Autogenerate feature gate code when adding it to metadata.yaml

[mx-psi]

Description

Generate code for feature gates that are described in metadata.yaml.

Link to tracking issue

Follows #14130

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.82%. Comparing base (5e41fb5) to head (ca60386).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14437      +/-   ##
==========================================
+ Coverage   91.80%   91.82%   +0.02%     
==========================================
  Files         677      677              
  Lines       42536    42610      +74     
==========================================
+ Hits        39050    39127      +77     
+ Misses       2425     2423       -2     
+ Partials     1061     1060       -1     

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