Implement async support for open_datatree #20451

Job	Run time
Event File	6s
detect ci trigger	6s
cache-pixi-lock	7s
ubuntu-latest \| test-py313	27s
macos-latest \| test-py311	27s
ubuntu-latest \| test-py311	25s
windows-latest \| test-py311	28s
macos-latest \| test-py313	27s
windows-latest \| test-py313	28s
ubuntu-latest \| test-py311-with-typing (mypy)	27s
ubuntu-latest \| test-py311-bare-min-and-scipy	26s
ubuntu-latest \| test-py311-bare-minimum	26s
ubuntu-latest \| test-py311-min-versions	27s
ubuntu-latest \| test-py313 (flaky)	27s
ubuntu-latest \| test-py313-with-typing	27s
ubuntu-latest \| test-py313-no-dask	26s
ubuntu-latest \| test-py313-no-numba	26s
	6m 33s

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