use datafusion::Unparser for converting datafusion Expr to SQL

[hozan23]

Hello,

This PR addresses issue #18.

It removes the basic unparser that was used to convert DataFusion's Expr into SQL and instead uses the implementation from DataFusion's unparser.

Additionally, this PR modifies the API for the SqlTable and SqlExec structs to accept a SQL Dialect, which is used for parsing.

[phillipleblanc]

Thanks for the PR!

[hozan23]

Hey @phillipleblanc,

I was also considering the need of Engine, but I'm thinking of wrapping the few Dialects offered by DataFusion with the current Engine enum we have. This way, we can still add some custom logic to these engines and not be restricted by the dialects offered by DataFusion. By doing this, we would no longer need to use Dialect directly, and we could remove it from SqlTable and SqlExec.

[phillipleblanc]

I think thats fine, but then we should have a mapping from Engine -> Dialect and not require the dialect to be passed in alongside the engine.

[phillipleblanc]

I pushed up a few commits to implement the Engine -> Dialect mapping and removed passing both Engine & Dialect.

[phillipleblanc]

Thanks @hozan23!

[hozan23]

Thanks for merging it @phillipleblanc