Create first class database management APIs in hanami-db

Right now all the methods for creating, migrating, deleting, etc. databases are spread across the various commands in Hanami-CLI.

We should make these real public APIs in the Hanami DB gem, and keep the db commands in the CLI as slim as possible, simply calling into these APIs.

At th same time time, we should look at directly talking to the database engine where possible, rather than shelling out to the database client CLIs (eg calling createdb and friends in Postgres’ case). Wrapping the ClIs is brittle, error prone and overly complicated to implement.