PostgreSQL CDC

[Jeffail]

I've merged the PgSQL repos for CDC into ./internal/impl/postgresql and have done some tidy up to get it (almost) compiling. There's a bunch of TODOs where we have panics and other things we wouldn't want to carry into a production plugin, but we're most of the way. There's also placeholders for *service.Logger which aren't hooked up yet.

[Jeffail]

@le-vlad It's probably easiest for you to kick off future work on a fork of this branch and then merge it in via a new PR.