Expand metadata to be able to track multi-column and group sort status + add lazyframe method to check whether a group or multicolumns are sorted

[kszlim]

Description

It would be nice to have the query plan also keep track of whether specific groups are sorted or not. I could see something like.

ldf.set_sorted(pl.col('a').over('b', 'c'))

And having a method like:

ldf = ldf.check_sorted(pl.col('a').over('b', 'c'))
ldf.collect() # this will return the original ldf, but will throw the group is unsorted.