Refactor how credentials works to be a bit more agnostic

[jwoertink]

Ref #415

This PR starts the work to make Avram a little more agnostic. Supporting alternate DBs like MySQL, or SQLite would still present a very hefty challenge since there's not a lot of people that contribute to this library, and I only use postgres. However, we've already had a few issues where CockroachDB differs from postgres even though it's uses the postgres protocol. With this change, we can now at least do something like

if AppDatabase.settings.engine.cockroachdb?
  exec("cockroach query")
else
  exec("postgres query")
end

A ton of things would need massive refactoring and thought before trying to support any non-postgres related DB like the PG.connect_listen or using PQ::Notification , etc.. I'll worry about those later.

For now, the big benefit is Avram::Credentials is now just a thing wrapper around URI so we no longer need to handle complex building logic.