Support writing CASA Tables from Arrow Tables

[sjperkins]

Initially only conversion of CASA Tables to Arrow Tables is supported. It may be worth supporting CASA Table writes.

There are three general cases to consider:

1. CREATE a new table. Table Descriptors and Data Managers must be synthesised from the schema of the supplied Arrow Table and the MSv2.0 spec.

   • Create and use JSON Table Descriptors and Data Managers #51

2. APPEND to an existing table. The Arrow Table will have to conform to the existing Table Descriptor and Data Managers.

3. UPDATE an existing table. The Arrow Table will have to conform to the existing Table Descriptor and Data Managers. New Table Descriptor and Data Manager entries must be synthesised for any columns that are not present on the CASA Table.

From experience with dask-ms, (3) is the most complex case. I think this should be low priority unless there is a demonstrably useful case for returning to the CTDS format.