Support arbitrary aggregate and window functions

[carlineng]

Implement the proposal outlined here: https://github.com/malloydata/whatsnext/blob/main/wns/WN-0003/wn-0003.md

From the issue above:

We can handle unknown scalar functions in the Malloy but we have to use an ugly workaround for aggregate functions and analytic/window functions are impossible today. This is a proposal would allow the use of unknown analytic and window functons.

In the most recent round of user feedback, we had a lot of people run into this issue.