RAD-171: Add statistics field to TVAC and FPS schemas

[PaulHuwe]

Resolves RAD-171

Closes #412

This PR adds a statistics schema to both FPS and TVAC.

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 95.45%. Comparing base (c1811ab) to head (820dd8d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #423      +/-   ##
==========================================
+ Coverage   95.38%   95.45%   +0.06%     
==========================================
  Files           4        4              
  Lines         195      198       +3     
==========================================
+ Hits          186      189       +3     
  Misses          9        9              

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[fjaviersanchez]

Looks good to me! Thank you