Add GREATEST(expr1, expr2, ... exprN) and LEAST(expr1, expr2, exprN)
functions which are in the family of CONDITIONAL functions.
Implementation follows PostgreSQL behaviour, so the functions return
`NULL` when all of their arguments evaluate to `NULL`.
Renamed `CoalescePipe` and `CoalesceProcessor` to `ConditionalPipe` and
`ConditionalProcessor` respectively, to be able to reuse them for
`Greatest` and `Least` evaluations. To achieve that `ConditionalOperation`
has been added to differentiate between the functionalities at execution
time.
Closes: #35878
3f7cae3f0d