WIP: FFI query planner

[milenkovicm]

Which issue does this PR close?

• Closes #.

Rationale for this change

This is WIP which exposes datafusion query planner as FFI interface,
I hope to split this PR into two separate once implementation is nailed

What changes are included in this PR?

• query planner moved to different module and interface changed (this will be subject of separate PR, once query planner part gets green light) there is question at which point Session should be downcast to SessionState. There are two approaches, non of which i particularly like
• FFI query planner implementation

There are few open questions

• is ffi planner module correct
• can we expose FFI_SessionRef as public

Are these changes tested?

There is one test, integration test to follow if this makes sese

Are there any user-facing changes?

yes query planner is moved to different module and signature changed

[milenkovicm]

it would be great if you could have a look @timsaucer, ffi/ part is important

[timsaucer]

Looking good! Instead of adding a lot of comments I put up this PR targeting your branch with some recommendations:

milenkovicm#1

[milenkovicm]

thanks @timsaucer