Refactor the ASDF extension into stnode

[WilliamJamieson]

This PR refactors the ASDF extension so that it exists entirely within the stnode sub-package because it is really an ASDF extension to handle serialization/deserialization of the stnode objects. This also makes the API for the extension explicitly private, as it should only be used by roman_datamodels to register itself as an ASDF extension.

Checklist

• Added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[WilliamJamieson]

This PR is based off the changes in #213, so it will remain a draft until that PR is merged.

[codecov]

Codecov Report

Patch coverage: 94.11% and project coverage change: -0.05 ⚠️

Comparison is base (2dd4d7f) 94.48% compared to head (628693f) 94.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
- Coverage   94.48%   94.44%   -0.05%     
==========================================
  Files          25       24       -1     
  Lines        1778     1782       +4     
==========================================
+ Hits         1680     1683       +3     
- Misses         98       99       +1     

Impacted Files	Coverage Δ
src/roman_datamodels/stnode/_converters.py	98.33% <92.30%> (-1.67%)	⬇️
src/roman_datamodels/datamodels/_core.py	89.67% <100.00%> (-0.06%)	⬇️
src/roman_datamodels/stnode/_integration.py	100.00% <100.00%> (ø)
src/roman_datamodels/stnode/_registry.py	100.00% <100.00%> (ø)

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[braingram]

One comment/question about NODE_EXTENSIONS and docs but looks good to me!