Adds support to `groups` unit type in window clauses

Spark currently support the two most common window functions unit types: rows and ranges. However, in PgSQL a new type was added: `groups`. 

According to this source, the difference is:
"""ROWS counts the exact number of rows in the frame.
RANGE performs logical windowing where we don’t count the number of rows, but look for a value offset.
GROUPS counts all groups of tied rows within the window."""