Initial attempt to get cubed working within xarray, as an alternative to dask.

• Closes Alternative parallel execution frameworks in xarray #6807, at least for the case of cubed
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst
• Correct type hints

I've added a manager kwarg to the .chunk methods so you can do da.chunk(manager="cubed") to convert to a chunked cubed.CoreArray, with the default still being da.chunk(manager="dask"). (I couldn't think of a better name than "manager", as "backend" and "executor" are already taken.)

At the moment it should work except for an import error that I don't understand, see below.

Fro cubed to work at all with this PR we would also need:

• Cubed to expose the correct array type consistently cubed Array vs cubed.CoreArray cubed-dev/cubed#123
• A cubed version of apply_gufunc [WIP] apply_gufunc cubed-dev/cubed#119 - implemented in Add apply_gufunc cubed-dev/cubed#149 🥳

To-dos for me on this PR:

• Re-route xarray.apply_ufunc through cubed.apply_gufunc instead of dask's apply_gufunc when appropriate,
• Add from_array_kwargs to opening functions, e.g. open_zarr, and open_dataset,
• Add from_array_kwargs to creation functions, such as full_like,
• Add store_kwargs as a way to propagate cubed-specific kwargs when saving to_zarr.

To complete this project more generally we should also:

• Have cubed.apply_gufunc support multiple output arguments Support multiple outputs in apply_gufunc cubed-dev/cubed#152
• Have a top-level cubed.unify_chunks to match dask.array.core.unify_chunks
• Write a test suite for wrapping cubed arrays, which would be best done via Public testing framework for duck array integration #6894
• Generalise xarray.map_blocks to work on cubed arrays, ideally by first rewriting xarray's implementation of map_blocks to use dask.array.map_blocks

cc @tomwhite