feat: add APPLY operator representation

[ashvina]

This PR adds a new relation type for APPLY operation. APPLY is similar to JOIN operator since it joins two table sources. However, in APPLY, the table source on the right depends on the result of computation of the table source on left. Since the right source cannot be computed without left, the right source is represented by an Expression in Substrait and not a relation. The right source will most frequently be a SubQuery.

Closes #357