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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.