Improved window and aggregate function signature

[timsaucer]

Which issue does this PR close?

None

Rationale for this change

This is a relatively minor change that allows users to pass a single expression for order_by and partition_by fields.

Without this change, if used improperly planning gets stuck in a loop. This improves ergonomics.

Old version required:

df.select(f.lag(column("a"), order_by=[column("b")])

and now you can also write

df.select(f.lag(column("a"), order_by=column("b"))

What changes are included in this PR?

Enhances the check to see if we pass a single expression. When this is true, wrap it in a list. Added unit tests.

Are there any user-facing changes?

This is a non-breaking signature change. It is backwards compatible.