Add support for handling zarr urls

[stephenworsley]

Closes #6967

[codecov]

Codecov Report

❌ Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.10%. Comparing base (28762c4) to head (beb1109).

Files with missing lines	Patch %	Lines
lib/iris/loading.py	0.00%	3 Missing and 1 partial ⚠️
lib/iris/io/__init__.py	25.00%	2 Missing and 1 partial ⚠️
lib/iris/fileformats/netcdf/saver.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7113      +/-   ##
==========================================
- Coverage   90.13%   90.10%   -0.04%     
==========================================
  Files          91       91              
  Lines       24957    24968      +11     
  Branches     4684     4688       +4     
==========================================
+ Hits        22496    22498       +2     
- Misses       1683     1689       +6     
- Partials      778      781       +3     

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