Add virtual_backend_kwargs argument to open_virtual_dataset

[TomNicholas]

For #243 (comment) we need to pass a fs_root kwarg down to some but not all readers. We will have other reader-specific kwargs like this come up in future too. To be in keeping with xarray.open_dataset's API these should be passed as a dict called reader_kwargs or backend_kwargs.

However there is an existing reader_options kwarg, which is meant to be specifically for fsspec options. Having both reader_options and reader_kwargs would be confusing, so should we
a) rename reader_options to reader_kwargs, having it include fsspec options too,
b) have reader_kwargs and fsspec_kwargs, deprecating reader_options?

So far this PR just adds a reader_kwargs option to all backends, but raises if it is used.

cc @norlandrhagen @sharkinsspatial

• Closes #xxxx
• Tests added
• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst
• New functions/methods are listed in api.rst
• New functionality has documentation

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 93.30%. Comparing base (3348670) to head (41ec08f).

Files with missing lines	Patch %	Lines
virtualizarr/readers/dmrpp.py	0.00%	2 Missing ⚠️
virtualizarr/readers/tiff.py	0.00%	2 Missing ⚠️
virtualizarr/readers/fits.py	50.00%	1 Missing ⚠️
virtualizarr/readers/hdf/hdf.py	50.00%	1 Missing ⚠️
virtualizarr/readers/hdf5.py	50.00%	1 Missing ⚠️
virtualizarr/readers/kerchunk.py	50.00%	1 Missing ⚠️
virtualizarr/readers/netcdf3.py	50.00%	1 Missing ⚠️
virtualizarr/readers/zarr_v3.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #315      +/-   ##
==========================================
+ Coverage   84.62%   93.30%   +8.68%     
==========================================
  Files          48       48              
  Lines        3362     3378      +16     
==========================================
+ Hits         2845     3152     +307     
+ Misses        517      226     -291     

Flag	Coverage Δ
unittests	93.30% <37.50%> (+8.68%)	⬆️

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[TomNicholas]

I've changed this PR to use virtual_backend_kwargs instead of reader_kwargs, which is closer to xarray's equivalent backend_kwargs, and also makes it less similar to reader_options. We should discuss this choice before implementing #245 though (as then it will become harder to change).