Opening via xarray backendentrypoint

[TomNicholas]

Some changes are needed in xarray to support using the xarray backend entrypoint system to open datasets from disk as ManifestArray-backed Variables just by passing a keyword arg to open_dataset/open_mfdataset. This requires dodging some internal array wrapping that occurs in the depths of xarray's backend machinery.

Originally posted by @TomNicholas in #14 (comment)

[TomNicholas]

This would be cool, but it's actually not needed for an MVP, because we have open_dataset_via_kerchunk.

[TomNicholas]

Trying to get this to work leads down rabbit holes like the one in pydata/xarray#8712

[TomNicholas]

Note that to get the syntax open_dataset(file, engine='virtualizarr', indexes={}) to work as a way of avoiding creating indexes does not actually require that indexes be added to xr.open_dataset upstream (xref pydata/xarray#6633). That's because if the kwarg is not recognized by xarray it should be passed on to our backend engine, which already does support an indexes kwarg.