Snapshot PR: Extract complex default impls from AggregateUDFImpl trait

Extract AggregateUDFImpl method that have complex implementations. This
makes it feasible to implement that trait with
`#[warn(clippy::missing_trait_methods)]` Clippy check enabled. That
check is very helpful when the type implementing `AggregateUDFImpl`
delegates many methods to another `AggregateUDFImpl` implementation and
therefore does not want to unconsciously rely on default implementation.
Sadly, Rust does not allow explicitly delegating to trait method's
default implementation. Extracting the logic to standalone functions
works around that limitation.

Author: findepi