Support for arbitrary aggregate functions

[NiallRees]

Right now it seems only possible to monitor the results of aggregate functions outlined in the column level monitors defined here: https://docs.elementary-data.com/guides/data-monitoring/tests-and-monitors-types

It would be cool to allow users to specify their own custom aggregate function. I'm thinking of a case where a user might be interested to monitor for anomalies on count_if(col_a > col_b) or anything else.