Add SQL query planning support for EXISTS subqueries

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
As a user of the DataFusion library for SQL parsing and logical query planning, I would like to be able to produce a valid logical plan from a SQL query containing an EXISTS subquery instead of failing with Unsupported ast node Exists.

Example query: SELECT * FROM a WHERE EXISTS (SELECT * FROM b WHERE a.id = b.id)

Describe the solution you'd like
The SQL should parse without error and produce a valid logical plan.

For reference, here is an example query and logical plan from Apache Spark:

scala> val df = spark.sql("SELECT * FROM t1 WHERE EXISTS (SELECT * FROM t2 WHERE t1.id = t2.id)")
df: org.apache.spark.sql.DataFrame = [id: int, name: string]

scala> df.queryExecution.logical 
res5: org.apache.spark.sql.catalyst.plans.logical.LogicalPlan =
'Project [*]
+- 'Filter exists#16 []
   :  +- 'Project [*]
   :     +- 'Filter ('t1.id = 't2.id)
   :        +- 'UnresolvedRelation [t2], [], false
   +- 'UnresolvedRelation [t1], [], false

Describe alternatives you've considered
None

Additional context
This is a subset of issue #123 which aims to add full support for EXISTS, including a physical plan.