RAD-159: Create the flux step schema

[stscieisenhamer]

Resolves RAD-159

Related PRs:

• roman_datamodel#332
• romancal#1160

This PR completes the schema setup for the new romancal FluxStep by adding it to the cal_step schema.

Checklist

• Schema changes discussed at RAD Review Board meeting
• Added entry in CHANGES.rst under the corresponding subsection
• Updated relevant roman_datamodels utilities and tests
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.38%. Comparing base (0fb1239) to head (c7aeb4d).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #395   +/-   ##
=======================================
  Coverage   95.38%   95.38%           
=======================================
  Files           4        4           
  Lines         195      195           
=======================================
  Hits          186      186           
  Misses          9        9           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[PaulHuwe]

LGTM, but we should wait on merging until this is discussed at the RAD meeting.

[PaulHuwe]

Resolves #394