Automatic Dask-Zarr chunk alignment

[josephnowak]

I'm still working on some details of this PR, but as it stands, it works and demonstrates an implementation that avoids breaking changes.

At this stage, my main concerns are:

1. Chunk Handling Utilities – It seems there are no utility functions for managing chunks, which results in possible duplicated code across the various backends. To address this, I created a class with several static methods, though I’m not convinced this is the best solution. I’m uncertain whether these functions should be placed in a separate file or incorporated into the existing common.py file for backends. Alternatively, should there be an external package that provides these chunk utilities? It’s possible the Dask team already has something similar implemented, but I haven't confirmed this yet.

2. Backend Test Organization – The test file for backends has grown significantly in size, making it harder to manage. It might be beneficial to start adding new tests in a separate file to improve maintainability.

I’d appreciate any feedback or suggestions regarding these concerns!

• Closes Automatic Dask-Zarr chunk alignment on the to_zarr method #9914
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst