Add ability to rollback migrations

[craiglabenz]

Is your feature request related to a problem? Please describe.

During development, it's common to iteratively build up a schema migration while adding multiple tables for a new feature. Currently, without an ability to rollback a migration, the only options are to create many small migrations (which greatly bloats a codebase) or manually remove any tables / columns added by the migration before deleting and recreating.

Describe the solution you'd like

It would be nice if migrations were generated with a rollback.sql file which could be executed if the developer indicates a specific target migration from their history.

Assuming a migration history of:

migrations/
  [timestamp-01]/
  [timestamp-02]-first-feature/

While working on that "first feature", a developer might realize they missed a column and could run:

$ dart bin/main.dart --apply-migrations --target=[timestamp-01]

Which would rollback [timestamp-02]-first-feature, allowing the developer to delete that directory, update their models' yaml files, and recreate the migration.

Additional considerations

Obviously, this feature would require additional logic to generate the DROP TABLE or ALTER SCHEMA [table] DROP [columns] queries, but I think this would be quite valuable.

[SandPod]

The developer experience around this could be significantly improved with a dedicated rollback feature.

Currently, support for rolling back migrations is done by utilizing repair migrations. This is documented here: https://docs.serverpod.dev/concepts/database/migrations#rolling-back-migrations

Repair migrations can target a specific migration version using the --version option documented here: https://docs.serverpod.dev/concepts/database/migrations#targeting-a-specific-migration

[AdamMusa]

Hey there, @craiglabenz and @SandPod I added some features here #2202