Using the shuffle primitive in Xarray

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dask recently added dask.array.shuffle to help with some classic GroupBy.map problems.

shuffle reorders the array so that all members of a single group are in a single chunk, with the possibility of multiple groups in a single chunk. I see a few ways to use this in Xarray:

1. GroupBy.shuffle() This shuffles and returns a new GroupBy object with which to do further operations (e.g. map).
2. Dataset.shuffle_by(Grouper) This shuffles, and returns a new dataset (or dataarray), so that the shuffled data can be persisted to disk or you can do other things later (xref #5674)
3. Use GroupBy.shuffle under the hood in DatasetGroupBy.quantile and DatasetGroupBy.median, so that the exact quantile always works regardless of chunking (right now we raise and error), this seems like a no-brainer.
4. Add either a shuffle kwarg to GroupBy.map and/or GroupBy.reduce or a new API (e.g. GroupBy.transform or GroupBy.map_shuffled) that will shuffle, then xarray.map_blocks a wrapper function that applies the Groupby on each block. This is how dask dataframe implements Groupby.apply

#9320 implements (1,2). (1) is mostly for convenience, I could easily see us recommending using (2) before calling the GroupBy.

Thoughts?

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Start by reading the dask.array.shuffle documentation and the GroupBy.map and GroupBy.reduce APIs described in the issue. Review issue #9320 to understand the implemented shuffle approaches, then determine which remaining API or quantile/median behavior is intended; done requires a settled design and corresponding implementation direction.