(R) Tidy up `data_source` plugin API

[cpsievert]

The current setup for implementing new data sources in R is a bit of a mess. As an implementer, you have to know a lot of details about internals (e.g., create_data_source() needs to create an object of class querychat_data_source?)

For ease of maintenance, we should consider following a similar class-based/interface/protocol pattern that the Python package has. This would be a breaking change that gets rid of create_system_prompt(), create_data_source(), execute_query(), etc., in favor of just one base class.