Subquery Unnesting Bugs/missing features

[jurplel]

• More testing (+ unit tests)
• Query plan correctness issues
  • Datafusion plan typing issues
• EXISTS clauses
• IN clauses
• ANY/ALL clauses
• IsNull/IsNotNull
• Correctness issue with COUNT(*) (requires adding left outer join to plan)
• Support more pushdowns (e.g. limit, joins)
• Optimizations from the paper are all missing (Out of scope?)

Here are some test subqueries from my development: https://pastebin.com/SJV9q03B