feat(r): Generic datasources

[npelikan]

This is a WIP but seems to basically work.

One question -- I retained the basic functionality for local data.frames (that is, df -> querychat -> df), where remote data sources instead return a dbplyr lazy tbl(), meant for chaining. Is this too confusing of a behavior split? Should local data.frames also return a tbl(), just now connected to duckdb?

A few immediate TODOs:

• add more documentation
• generate some examples
• create shinytests
• validate this works on more than just sqlite

[npelikan]

(Changing the target of this to main so it merges cleanly)

[jcheng5]

I'm really sorry, this is still waiting on me, isn't it? I'm going to book some time for us to discuss in realtime if that's cool with you.

[npelikan]

Alright, I think this is ready to merge @jcheng5

[jcheng5]

Shooot, there is one problem left. We're now using dplyr::tbl(conn, dplyr::sql(query)) to actually perform the SQL queries. The problem is that dplyr assumes that second argument is a table name, not a whole SQL query. So little things that are valid for SQL queries, break; for example, a trailing semicolon, or ending on a SQL comment. You end up seeing pretty confusing error messages with unrecognizable SQL like:

SELECT * FROM (SELECT * FROM iris --test) AS `q01` WHERE (0 = 1)

I'll ask Hadley what the right way to do this is (if there is one).