Incorporate unified Aggregate / Window function builder

[Michael-J-Ward]

@timsaucer contributed a unified AggregateUDF/WindowUDF builder to upstream datafusion: apache/datafusion#11550

We should expose that and leverage that to clean up the functions.rs APIs.

[timsaucer]

I'll take this one on. I have a template put up based on your 41.0.0 branch where I added the first. The rest should be fairly straightforward I think.