Add support for exists predicate

[beikov]

Migrated from Blazebit/blaze-notify#22

I would count the exists predicate into the COLLECTION predicate types class. The use could look like this EXISTS (document.users.email = '[email protected]' AND document.users.active)
The parenthesis are optional for a simple predicate but necessary if composite predicates should be considered in the exists predicate.
The semantics of this could be WHERE EXISTS(SELECT 1 FROM document.users u WHERE u.email = '[email protected]' AND u.active = TRUE).
I would work on the query translation to make this work properly as I think this will be a very helpful tool for many use cases.