Support `EXISTS (subquery)` operator

[MuhannadAlrusayni]

Hi, I'm trying to do simple query and use EXISTS (subquery) operator, but it looks it's not supported yet in sea-query.

.e.g:

select exists(select 1 from accounts where email = $1 limit 1) as "exists!"

[tyt2y3]

I think it can be done like

Query::select().expr(Expr::cust("COUNT(*) AS count")).from_subquery(...);

[MuhannadAlrusayni]

Yeah that's work too, but I'm not sure about the query performance count() vs `exists(). I'm no expert in query performance things.

And I think we should have exists() sooner or later.

[penso]

+1 on having exists()

[ikrivosheev]

I will look at the issue

[ikrivosheev]

Hello! I create PR: #118.