SQL: Add GREATEST and LEAST functions

[matriv]

Add GREATEST(expr1, expr2, ... exprN) and LEAST(expr, expr2,... exprN) functions.

The first one returns the argument with the largest value.
The second on returns the arguments with the smallest value.
Both skip any null arguments in their list and they return null if all arguments are null.

Relates to #34956

[elasticmachine]

Pinging @elastic/es-search