[SPARK-14785] [SQL] Support correlated scalar subqueries

## What changes were proposed in this pull request?
In this PR we add support for correlated scalar subqueries. An example of such a query is:
```SQL
select * from tbl1 a where a.value > (select max(value) from tbl2 b where b.key = a.key)
```
The implementation adds the `RewriteCorrelatedScalarSubquery` rule to the Optimizer. This rule plans these subqueries using `LEFT OUTER` joins. It currently supports rewrites for `Project`, `Aggregate` & `Filter` logical plans.

I could not find a well defined semantics for the use of scalar subqueries in an `Aggregate`. The current implementation currently evaluates the scalar subquery *before* aggregation. This means that you either have to make scalar subquery part of the grouping expression, or that you have to aggregate it further on. I am open to suggestions on this.

The implementation currently forces the uniqueness of a scalar subquery by enforcing that it is aggregated and that the resulting column is wrapped in an `AggregateExpression`.

## How was this patch tested?
Added tests to `SubquerySuite`.

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #12822 from hvanhovell/SPARK-14785.