ANY_VALUE aggregate

The SQL:2023 Standard defines a new aggregate named ANY_VALUE. It
returns an implementation-dependent (i.e. non-deterministic) value from
the rows in its group.

PFA an implementation of this aggregate.

Ideally, the transition function would stop being called after the first
non-null was found, and then the entire aggregation would stop when all
functions say they are finished[*], but this patch does not go anywhere
near that far.

This patch is based off of commit fb958b5da8.

[*] I can imagine something like array_agg(c ORDER BY x LIMIT 5) to get
the top five of something without going through a LATERAL subquery.
--
Vik Fearing