Support passing just name and data_type to add_columns

[wjones127]

If you just want to add a column, but not fill it in immediately, the API makes it possible, but it is awkward.

table.add_columns({"new_column": "cast(NULL as int64)"})

It would be nicer if a user could use

table.add_columns([pa.field("new_column", pa.int64())])

There's an upstream issue that would support this: #3044

We could also make a hacky implementation for now that does the conversion from a field to a cast(NULL as ____) expression.