add grating keyword to barshadow schema

[braingram]

CRDS now uses GRATING for barshadow reference files.

This PR updates the barshadow schema to include the grating keyword schema.

See: https://jwst-crds.stsci.edu/browse/jwst_nirspec_barshadow_0007.rmap

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 (b8ed8be) 64.84% compared to head (ef6717c) 64.84%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #260   +/-   ##
=======================================
  Coverage   64.84%   64.84%           
=======================================
  Files         103      103           
  Lines        5694     5694           
=======================================
  Hits         3692     3692           
  Misses       2002     2002           

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[tapastro]

LGTM