Expose DataFusion Dataframe API

[cbiggar]

It appears as though the Java bindings only expose a SQL string as a query interface.

This makes programmatic querying of the DataFusion engine hard as it would rely on translating our internal dataframe-like AST into a SQL string first. This feels brittle as it introduces a whole class of bugs related to SQL syntax errors which aren't a concern when translating from an internal dataframe representation to Datafusion's dataframe representation.

I can see that the Rust Native DataFusion API contains a Dataframe API.

Would you consider exposing this in the java bindings too?