Add existance options to migrations

This particularly helps migrating to Avram with an existing database

- [ ] create table if not exists
- [ ] drop table if exists
- [ ] add column if not exists
- [ ] drop column if exists

Locate the migration implementations for creating and dropping tables and adding and dropping columns, along with any existing migration tests. Start by tracing how these four operations generate PostgreSQL statements. Done means each listed existence option is supported and covered by tests for an existing or missing database object.

Written by the indexing model from the issue text.