DM-48282: Refactor obscore to allow subclasses

[timj]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

Attention: Patch coverage is 68.86792% with 33 lines in your changes missing coverage. Please review.

Project coverage is 89.46%. Comparing base (524ffd8) to head (a7af72e).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ython/lsst/daf/butler/registry/obscore/_records.py	67.64%	23 Missing and 10 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1142      +/-   ##
==========================================
- Coverage   89.49%   89.46%   -0.04%     
==========================================
  Files         366      366              
  Lines       49018    49068      +50     
  Branches     5941     5950       +9     
==========================================
+ Hits        43870    43897      +27     
- Misses       3733     3753      +20     
- Partials     1415     1418       +3     

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[andy-slac]

Looks good. One thing that I'm not sure about is whether it's easier to specify record factory type in obscore config vs. entry point.