Expose user defined functions in the FFI

[timsaucer]

Is your feature request related to a problem or challenge?

By adding the foreign function interface to this project, we exposed a host of features to end users. It allows for easy integration of DataFusion libraries across projects, specifically impacting datafusion-python.

I recommend that we work to expose scalar, aggregate, and window user defined functions via FFI. This is to support the issue apache/datafusion-python#1017

Describe the solution you'd like

Follow the existing pattern within the datafusion repository and expose these types of user defined functions.

• scalar
• aggregate
• window
• table

Note: The table functions are not currently in use in datafusion-python, but it would be a good addition.

Describe alternatives you've considered

For the datafusion-python project, we can work around via python calls but this would be a far more elegant approach in my opinion.

Additional context

Once we have these exposed, we could expose the function registry, which would get us very close to a FFI SessionContext. That would have impacts on the ballista and datafusion-ray projects.