Support xarray grouper objects in xarray interface

[dcherian]

I think we need a new much improved interface that only uses Grouper objects following the API in pydata/xarray#6610 (comment), and potentially using a flox accessor (this may be a lot quicker than adding multiple grouper support to xarray directly)

We will need to make the Grouper objects public in Xarray first, but that's not a bad blocker.

data.flox.groupby({
	"x0": xr.BinGrouper(bins=pd.IntervalIndex.from_breaks(coords["x_vertices"])),  # binning
    "y": xr.UniqueGrouper(labels=["a", "b", "c"]),  # categorical, data.y is dask-backed
    "time": xr.TimeResampleGrouper(freq="MS")
	},
)