`AggregateUDF` / UDAF Support

# What
Support [`AggregateUDF`](https://docs.rs/datafusion/latest/datafusion/logical_expr/struct.AggregateUDF.html).

# Why
Allows people to write their own aggregation functions.

# How
This likely needs DataFusion support for async aggregate UDFs similar to [`AsyncScalarUDF`](https://docs.rs/datafusion/latest/datafusion/logical_expr/async_udf/struct.AsyncScalarUDF.html) first.