SQL: add DATE_TRUNC function

[astefan]

The DATE_TRUNC function is used to truncate date/datetimes to specified precision.
https://www.w3resource.com/PostgreSQL/date_trunc-function.php

[elasticmachine]

Pinging @elastic/es-search

[jbonn360]

@astefan does this need approval? I can work on this

[astefan]

@jbonn360 thank you for your interest in ES-SQL. We do appreciate it.
We've already started looking at this implementation and it's not that trivial. If you maybe would like to have a look at a more simple issue to start with, that would be preferable.

[jbonn360]

@astefan ok cheers

[matriv]

@jbonn360 Once again, thank you for your interest to contribute to our project.
If you'd like you could give a go on a functionality extension of DATE_TRUNC to include intervals: #46632