Allow .sql files for migrations

[joseferben]

Currently it is only possible to register migrations as data.

It should be possible to create 1_up.sql and 1_down.sql and so on in database/. The migrations should be registered under a configurable namespace, the default could be something like app.

We don't want timestamps in the migration file names to end up with merge conflicts that need to be resolved.

Consider implementing this first: https://github.com/oxidizing/sihl/issues/387