[Postgres] Test and document support for YugabyteDB

[pythoneer]

As a sibling suggestion to #198 . I would be willing to help.

[mehcode]

From a skim it seems like it should "just work" with the postgres driver. We should test and document this.

Next, it seems like there is actually additional (optional) complexity to the protocol where it would make sense to have a sqlx::yogabyte module that is based on the sqlx::postgres module. That should be split into a low priority task. I don't see us circling back to enhance support for spin off databases for a while. I'd love to accept a PR here though.