JP-3520: Add ngroups to mask schema

[braingram]

Addresses JP-3520

Closes [spacetelescope/jwst#8219]

crds added 'ngroups' as a parkey for mask reference files. This is causing test failures (that check the schema and parkeys).

See the test failure here: https://github.com/asdf-format/asdf/actions/runs/7629799922/job/20784057576?pr=1739#step:10:718

This PR adds 'ngroups' to the mask schema to bring these two projects into alignment.

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (051be5f) 64.84% compared to head (a030cda) 64.84%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #249   +/-   ##
=======================================
  Coverage   64.84%   64.84%           
=======================================
  Files         103      103           
  Lines        5694     5694           
=======================================
  Hits         3692     3692           
  Misses       2002     2002           

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[tapastro]

Great breadcrumb!