DM-38694: Add StorageClassDelegate.copy() method

[timj]

This allows for a future where in-memory datastore can decide whether to do copying when returning an object.

Checklist

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

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3e38a7b) 87.76% compared to head (1a976e6) 87.77%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #819   +/-   ##
=======================================
  Coverage   87.76%   87.77%           
=======================================
  Files         268      268           
  Lines       35246    35262   +16     
  Branches     7420     7421    +1     
=======================================
+ Hits        30935    30951   +16     
  Misses       3154     3154           
  Partials     1157     1157           

Impacted Files	Coverage Δ
...ython/lsst/daf/butler/core/storageClassDelegate.py	49.59% <100.00%> (+2.58%)	⬆️
tests/test_storageClass.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[erykoff]

I think we want to test the alternate branch (failure) as well as the success branch.