AggregateUDF expression API design

[jayzhan211]

File an issue to track the design of UDAF API

Perhaps something like

// form `FIRST_VALUE(a ORDER BY b)`
let agg_expr = AggregateUDF::call(first_value())
  .args(col("a"))
  .order_by(vec![col("b"))
  .build()?;

Originally posted by @alamb in https://github.com/apache/datafusion/pull/10492#discussion_r1599967446

[alamb]

related to https://github.com/apache/datafusion/issues/6747 (for window APIs)

[alamb]

https://github.com/apache/datafusion/pull/10560 is looking sweet to me