refactor(storage)!: ReadObjectResponse uses pimpl

[dbolduc]

Part of the work for #2041, reverts #2931

Restore ReadObjectResponse to be a concrete object.

Implement it using a pimpl pattern. This will let us inject a fake implementation when we need to mock it as a return type.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.50%. Comparing base (c5258b7) to head (61b7468).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3137   +/-   ##
=======================================
  Coverage   96.50%   96.50%           
=======================================
  Files         107      108    +1     
  Lines        4402     4407    +5     
=======================================
+ Hits         4248     4253    +5     
  Misses        154      154           

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[dbolduc]

Had to rebase, PTAL