Context to allow providing custom data sources, functions, etc.

[nevi-me]

I was trying to keep things simple, avoiding a context, but the typesystem won't allow me to have custom sources without some stateful place to register them.

Nothing special here, so I'll take a cue from DataFusion.
What I'm interested in bikeshedding here is how to create an expressive API that allows data sources to declare their capabilities (e.g. I can pushdown sorts, filters, projections).

If I can get it to work, I'd want to contribute it to DataFusion, as that's where I think a de-facto Rust data analysis library should be.