Allow passing in a string transform for tbl.update_spec().add_field()

[Fokko]

Feature Request / Improvement

Today you have to pass in an instance of the transform:

with  tbl.update_spec() as upd:
    upd.add_field("tpep_pickup_datetime", DayTransform())

Instead, I think it would be great to also pass in the name:

with  tbl.update_spec() as upd:
    upd.add_field("tpep_pickup_datetime", "day")

This should be pretty straightforward using the parse_transform function.