Make zarr ArrayWriter customizable when writing dataset to zarr

[AchilleAsh]

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

The dump_to_store function was already exposing the ability to specify a writer, but this was never exposed on any of the to_zarr function, and was simply hardcoded to ArrayWriter.

This PR adds the optional keyword argument to override it, and just pass it to the dump function.

Let me know if you need me to add documentation on it.

Thanks!

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[shoyer]

Hi @AchilleAsh, what use cases did you have in mind here? ArrayWriter was not intended to be a public API.