[WIP] JDBC source

This is still a work in progress, but I wanted to open the pull request early so you could comment on preferred ways to do the implementation.

Right now setting up a connection and listing tables is working, I still need to work with the source some more to verify everything else is working and make the source testable.

Testing should be pretty easy actually, since there are JDBC drivers for both mysql and postgres, so we can just reuse the databases for those using their drivers.
These docs need a bit of an update.