DM-35741: Add some improvements to StorageClass

[timj]

• StorageClass.is_type
• StorageClassFactory iterator

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #719 (04b51cb) into main (018065d) will decrease coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #719      +/-   ##
==========================================
- Coverage   84.59%   84.44%   -0.16%     
==========================================
  Files         243      243              
  Lines       31922    31637     -285     
  Branches     5990     5339     -651     
==========================================
- Hits        27006    26715     -291     
  Misses       3746     3746              
- Partials     1170     1176       +6     

Impacted Files	Coverage Δ
python/lsst/daf/butler/core/storageClass.py	93.84% <100.00%> (+0.26%)	⬆️
tests/test_storageClass.py	98.98% <100.00%> (+0.07%)	⬆️
python/lsst/daf/butler/registry/_collectionType.py	79.16% <0.00%> (-2.98%)	⬇️
...ython/lsst/daf/butler/core/datasets/association.py	86.66% <0.00%> (-2.23%)	⬇️
python/lsst/daf/butler/formatters/pickle.py	69.23% <0.00%> (-2.20%)	⬇️
...on/lsst/daf/butler/core/dimensions/construction.py	80.39% <0.00%> (-2.07%)	⬇️
python/lsst/daf/butler/transfers/_context.py	85.45% <0.00%> (-1.82%)	⬇️
python/lsst/daf/butler/core/ddl.py	81.53% <0.00%> (-1.80%)	⬇️
python/lsst/daf/butler/_deferredDatasetHandle.py	77.77% <0.00%> (-1.54%)	⬇️
python/lsst/daf/butler/formatters/yaml.py	62.74% <0.00%> (-1.41%)	⬇️
... and 79 more

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