JP-4091 adding MIRI WFSS background subtraction

[jemorrison]

This is a work in progress.

Resolves JP-4091

This PR adds the hooks in background_step to subtract a MIRI WFSS reference file.
The current MIRI WFSS background reference file in CRDS needs to be updated.

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
    • if your change breaks step-level or public API (as defined in the docs), also add a changes/<PR#>.breaking.rst news fragment
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (bbf1e5f) to head (c85d7ff).
⚠️ Report is 67 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10241      +/-   ##
==========================================
+ Coverage   85.98%   86.51%   +0.53%     
==========================================
  Files         368      366       -2     
  Lines       38457    38458       +1     
==========================================
+ Hits        33066    33271     +205     
+ Misses       5391     5187     -204     

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