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.

[guptaakashdeep]

@Fokko I have made changes to support this and has also added a test to test this for one of the transform passed as string name.

I had to re-position the parse_transform method to change the return type of the method from Any to Transform[Any, Any] because mypy wasn't happy after we made Transform type as Union[str, Transform]

Can you please review the changes once whenever you get chance?