ES|QL: add mv_values function

[astefan]

Description

Like many other aggregate functions, values must have a sister function: mv_values. This is needed for properly handling aggregations over constants work.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[ivancea]

Looks like we already have MV_DEDUPE, which apparently does the same

Edit: Closing in favor of mv_dedupe