aggregated_by preserving masked aux coords

[bjlittle]

🚀 Pull Request

Description

Addresses first of two parts of #6473 i.e., supporting masked aux coords for Cube.aggregated_by.

Follow-on separate pull-request will address supporting masked aux coords for Cube.collapsed.

• Add unit test coverage

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (57a316c) to head (7d5caea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6706   +/-   ##
=======================================
  Coverage   90.29%   90.29%           
=======================================
  Files          91       91           
  Lines       24467    24472    +5     
  Branches     4568     4569    +1     
=======================================
+ Hits        22092    22097    +5     
  Misses       1607     1607           
  Partials      768      768           

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