Some backends are subclassed and others are monkey patched - better to do it one way #1771

Is your feature request related to a problem? Please describe.
This is code refactoring, clarity and improvement

What version of DVT are you using?
8.7

What type of connections are you using for source and target?
Postgres

Describe the solution you'd like
DVT uses backend in two different ways

1. The Oracle, MSSQL model - where the a Backend child class with BaseAlchemyBackend as the base class
2. The Postgres, MySQL model - where the ibis backend is monkey patched to add libraries.

Both models provide similar functionality, customized do_connect, list_primary_keys, raw_metadata, is_char_padded etc. The first appears cleaner, the second is difficult to understand.

Better if we go with one model, perhaps the first - thoughts?