add schema and model for nrm reference file

[braingram]

This PR adds support (a schema and model) for the new NIRISS nrm reference file: https://jwst-crds-test.stsci.edu/browse/jwst_niriss_nrm_0002.fits

Local tests were performed to confirm that this file loads and the nrm array is available at the nrm attribute:

>> import stdatamodels.jwst.datamodels as dm
>> m = dm.open('jwst_niriss_nrm_0002.fits')
>> assert isinstance(m, dm.NRMModel)
>> m.nrm
array([[0., 0., 0., ..., 0., 0., 0.],
       [0., 0., 0., ..., 0., 0., 0.],
       [0., 0., 0., ..., 0., 0., 0.],
       ...,
       [0., 0., 0., ..., 0., 0., 0.],
       [0., 0., 0., ..., 0., 0., 0.],
       [0., 0., 0., ..., 0., 0., 0.]], dtype='>f4')

The local testing is important as this file only exists on https://jwst-crds-test.stsci.edu/ and the CI uses the non-test public crds: https://jwst-crds.stsci.edu/

Checklist

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

Project coverage is 64.87%. Comparing base (4d7c3a6) to head (591ae0d). Report is 1 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #253 +/- ## ========================================== + Coverage 64.84% 64.87% +0.03% ========================================== Files 103 104 +1 Lines 5694 5699 +5 ========================================== + Hits 3692 3697 +5 Misses 2002 2002 ```

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