ESQL: mv_sort should check and only allow certain values for order parameter

[astefan]

Description

Right now, mv_sort allows any kind of value to be placed in the optional order parameter.
This makes it hard for users to understand if the intention they had with the query is being met or not. Plus, if we change in the future the values we now allow in order to something else, the current behavior of silently (no error, no warning header) ignoring the faulty value and defaulting to "asc" will be a hidden breaking change.

[elasticsearchmachine]

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