SQL: consider adding conditional expression support

[AdiJurca]

At the moment there is no way of doing conditional expressions in SQL.

Having at least the CASE statement and COALESCE as described here

https://www.postgresql.org/docs/9.4/static/functions-conditional.html

would be of great help.

[elasticmachine]

Pinging @elastic/es-search-aggs

[astefan]

This is already implemented by #41349 and #35253.