Add Expr::Exists to logical plan

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I would like support for EXISTS and the first step is to add a new variant Expr::Exists.

Describe the solution you'd like
Add a new variant Expr::Exists and support plans with EXISTS being created via LogicalPlanBuilder`.

Describe alternatives you've considered
None

Additional context
None