Support Per-node DataTree chunking

[sjperkins]

• ClosesPer-node DataTree chunking #9634
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

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[sjperkins]

Some initial work to support per-node DataTree chunking. It might be this easy (I'm guessing because of all the work @keewis did in #9660), but I've almost certainly missed some subtleties.

One thing that occurred to me was that it's probably worth supporting a default chunk entry, perhaps keyed with "".

I still need to get my head around the mypy errors, but i's EOW for me.