Usage of sqlx migration with postgres

[xanderio]

It is currently not possible to use apalis on the same postgres database as the main application.

This is do to the sqlx Migrator currently not support multiple applications using the same database. As the postgres migrations are already using a separate schema, it would be quite nice to be able to setup apalis in the same database as the main application it self.

I currently see two options:

1. Users have to copy our migrations into there sqlx migration folder.
2. We don't use the sqlx migrator for postgres until sqlx supports mutliple applications in the same database.

This isn't a problem for sqlite or mysql as either of these support postgresql like schemas.

See: launchbadge/sqlx#1698

[geofmureithi]

This can be solved with an approach similar to #81