We are moving to using `DatabaseConfig` objects everywhere inside of
Rails instead of passing around Hash objects.

This PR moves to using `DatabaseConfig` objects inside of the individual
database tasks adapters. In the interest of not breaking existing
adapters, we've introduced an upgrade path by which adapters can get a
hold of database configuration objects instead of Hashes by implementing
a method `self.using_database_configurations?`

Co-authored-by: eileencodes <eileencodes@gmail.com>