JP-3078: Add support custom reference WCS for the resample step

[mcara]

Resolves JP-3078

This PR addsoutput_wcs argument to the resample_step and resample_spec_step that allows users to provide their own reference GWCS to the resample step.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

Regression test(s):

https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/554/

[codecov]

Codecov Report

Base: 78.01% // Head: 78.58% // Increases project coverage by +0.56% 🎉

Coverage data is based on head (c989a20) compared to base (04688a7).
Patch has no changes to coverable lines.

❗ Current head c989a20 differs from pull request most recent head 04ef8dc. Consider uploading reports for the commit 04ef8dc to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7442      +/-   ##
==========================================
+ Coverage   78.01%   78.58%   +0.56%     
==========================================
  Files         453      455       +2     
  Lines       36830    39200    +2370     
==========================================
+ Hits        28732    30804    +2072     
- Misses       8098     8396     +298     

Flag	Coverage Δ
nightly	78.57% <0.00%> (+0.56%)	⬆️
unit	51.35% <0.00%> (+0.90%)	⬆️

Impacted Files	Coverage Δ
jwst/datamodels/schema.py	6.16% <0.00%> (-93.84%)	⬇️
jwst/datamodels/asn.py	51.51% <0.00%> (-48.49%)	⬇️
jwst/datamodels/tsophot.py	51.72% <0.00%> (-48.28%)	⬇️
jwst/datamodels/multiexposure.py	57.89% <0.00%> (-42.11%)	⬇️
jwst/datamodels/nirspec_flat.py	60.71% <0.00%> (-39.29%)	⬇️
jwst/datamodels/multiextract1d.py	61.53% <0.00%> (-38.47%)	⬇️
jwst/datamodels/multispec.py	66.66% <0.00%> (-33.34%)	⬇️
jwst/datamodels/multicombinedspec.py	66.66% <0.00%> (-33.34%)	⬇️
jwst/datamodels/wavemap.py	69.23% <0.00%> (-30.77%)	⬇️
jwst/datamodels/spectrace.py	69.23% <0.00%> (-30.77%)	⬇️
... and 210 more

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[hbushouse]

Looks OK to me. Updated docs seem to build correctly.