JSON Schemas: Add Options Pages schema

[priethor]

What

Part of #162; adds JSON Schema validation for SCF UI Options Pages, continuing the schema implementation work from post types and taxonomies.

[github-actions]

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Props priethor.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 9.37%. Comparing base (342675c) to head (5018c05).

Additional details and impacted files

@@           Coverage Diff           @@
##             trunk    #270   +/-   ##
=======================================
  Coverage     9.37%   9.37%           
  Complexity    4288    4288           
=======================================
  Files          266     266           
  Lines        33159   33159           
=======================================
  Hits          3108    3108           
  Misses       30051   30051           

Flag	Coverage Δ
e2e	21.83% <ø> (ø)
phpunit	6.08% <ø> (ø)

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