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