feat(migrations): allow a 3rd party migrator to be configured

[hamptokr]

Thanks for the wonderful library!

I'm working on an external package that will provide a MySQL adapter for Oban (#836) and I need to be able to override the migrator used. This change will allow a migrator key in the Repo's config to allow for that:

config :oban_mysql, ObanMySQL.Test.MySQLRepo,
  priv: "test/support/mysql",
  url: System.get_env("DATABASE_URL") || "mysql://root@localhost/oban_mysql_test",
  migrator: ObanMySQL.Migrations.MySQL,
  pool: Ecto.Adapters.SQL.Sandbox

If you have a different name suggestion or have a different approach in mind, please let me know! 🙂

The very very WIP repo is over here: https://github.com/hamptokr/oban_mysql
With this change I was able to get the migrations running and the migration tests passing. I'm just starting to port over the engine and its test suite.