DataFunction support for external data (in-db)

The current DataFunction paradigm only allows for sending data from and to the Spotfire in-memory data engine.

For a Spotfire user, a Spotfire data table is a Spotfire data table, whether it is external or not. For data science and analysis applications connecting to big data, not being able to use the super powerful and flexible data function paradigm risks holding Spotfire back for serious data analysis applications where data sizes only tend to grow,

Proposal to let the DataFunction use push-down calculations (python scripts or similar instructions) to external data sources (already possible, but input methodology is limited to the in-memory paradigm) and create Spotfire data table definitions using connections to external data for reading results.

This last part is what is mostly lacking in order to perform calculations using data functions that leaves results in external systems.