JP-3528: Add "IMAGER" as allowed value for MRSPRCHN keyword

[penaguerrero]

Resolves JP-3528 Closes spacetelescope/jwst#8252

This PR adds "IMAGER" as another allowed value for the "MRSPRCHN" keyword, in order to support proper handling of MIRI MRS and Imager exposures done in parallel.

Checklist

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

Comparison is base (b7c7bcd) 64.84% compared to head (d770405) 64.84%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #259 +/- ## ======================================= Coverage 64.84% 64.84% ======================================= Files 103 103 Lines 5694 5694 ======================================= Hits 3692 3692 Misses 2002 2002 ```

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[braingram]

The CI failures here are unrelated (they're due to a CRDS update addressed in: https://github.com/spacetelescope/stdatamodels/pull/260).