Add support for writing MSv2 MAIN table columns

[sjperkins]

It would be useful for xarray-ms to have some support for writing back to CASA MSv2.

This enhancement covers the following cases:

• Writing back to an existing MAIN table column.
• Adding a new column to the MAIN table.

It does not cover writing an entirely new MSv2 dataset as this is a large undertaking.

arcae tracking issue:

• Support addition of columns ska-sa/arcae#119

/cc @JSKenyon @landmanbester @o-smirnov @bennahugo