Reimplement DataTree aggregations #14771

Job	Run time
detect ci trigger	3s
Event File	2s
ubuntu-latest py3.10	9m 24s
ubuntu-latest py3.12	10m 3s
macos-latest py3.10	6m 53s
macos-latest py3.12	6m 55s
windows-latest py3.10	15m 59s
windows-latest py3.12	16m 23s
ubuntu-latest py3.10 bare-minimum	1m 51s
ubuntu-latest py3.10 min-all-deps	6m 33s
ubuntu-latest py3.12 all-but-numba	7m 27s
ubuntu-latest py3.11 all-but-dask	7m 13s
ubuntu-latest py3.12 flaky	1m 43s
	1h 30m 29s

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