Encode Dataset attributes containing Datasets as JSON

[sjperkins]

MSv4 specifies that the main_xds Dataset contains a number of Datasets in the attributes.

• https://docs.google.com/spreadsheets/d/14a6qMap9M5r_vjpLnaBKxsR9TF4azN5LVdOxLacOX-s/edit?gid=1049651074#gid=1049651074

For example, antenna_xds and source_xds.

xarray's default xarray.Dataset.to_zarr refuses to write these to disk as they are compound objects.

In this PR, we encode Datasets in attributes as JSON and use xds_{from,to}_zarr and xdt_{from,to}_zarr methods to perform the encoding/decoding of these attributes.

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📚 Documentation preview 📚: https://xarray-ms--10.org.readthedocs.build/en/10/