Add asynchronous load method

[TomNicholas]

Adds an .async_load() method to Variable, which works by plumbing async get_duck_array all the way down until it finally gets to the async methods zarr v3 exposes.

Needs a lot of refactoring before it could be merged, but it works.

• Closes Add an asynchronous load method? #10326
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

API:

• Variable.load_async
• DataArray.load_async
• Dataset.load_async
• DataTree.load_async
• load_dataset?
• load_dataarray?

[TomNicholas]

Notes to self:

• Try to consolidate indexing tests with those in test_variable.py, potentially by defining a subclass of Variable that only implements async methods
• Use create_test_data, write to a zarr (memory)store, and open lazily - this will help test decoding machinery.
• Raise informative error if you try to do o/v-indexing with a version of zarr that's too old? Or just fall back to blocking in that case...

[TomNicholas]

The changes needed in zarr-python have just been merged upstream (but not yet released).

[TomNicholas]

Upstream-dev tests are passing. But they all need to pass otherwise these new methods will error on zarr-python<=3.1.1.