DM-4551: Add support for storage class conversion on get from in-memory dataset handle

[timj]

This matches the change to DeferredDatasetHandle (lsst/daf_butler#737)

@TallJimbo you probably should look at this and the new deferred dataset handle changes in daf_butler given I had completely forgotten about this when you did your original review.

Checklist

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

[codecov]

Codecov Report

Base: 81.92% // Head: 81.99% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (f9b04e8) compared to base (96a1ab1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #283      +/-   ##
==========================================
+ Coverage   81.92%   81.99%   +0.06%     
==========================================
  Files          56       56              
  Lines        6103     6126      +23     
  Branches     1114     1118       +4     
==========================================
+ Hits         5000     5023      +23     
  Misses        873      873              
  Partials      230      230              

Impacted Files	Coverage Δ
python/lsst/pipe/base/_dataset_handle.py	100.00% <100.00%> (ø)
tests/test_dataset_handle.py	97.10% <100.00%> (+0.67%)	⬆️

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