Add xarray.open_zarr_async()

[davidbrochart]

Description

It looks like Zarr has an async API for opening a store, but xarray doesn't. There is only the sync open_zarr(), which calls the sync Zarr API which runs the async API in an event loop in a separate thread. I understand that this is not in the critical path (it's metadata), but that prevents using xarray in WASM Python, since threads are not supported there.

Checklist

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

AI Disclosure

• This PR contains AI-generated content.

  • I have tested any AI-generated content in my PR.

  • I take responsibility for any AI-generated content in my PR.

    Tools: Codex
    Prompt: "it looks like xarray's open_zarr is a synchronous function that eventually calls zarr's sync API that runs the async API in an event loop in a separate thread, so that won't work for WASM python where threads are not supported. is there a way to open a zarr store asynchronously in xarray? if not, could you make changes that add the async equivalent to open_zarr?"

[davidbrochart]

What do you think @shoyer ?