Feature request: Repeatable migrations for managing functions and views

### Feature description

Flyway has the concept of [repeatable migrations](https://documentation.red-gate.com/fd/repeatable-migrations-273973335.html) that are (re-)applied to a database on `migrate` every time their checksum changes.

This differs from the 'current' migration concept in Graphile Migrate, which as far as I understand is purely for development purposes.

Repeatable migrations are very useful for managing database objects whose definition can then simply be maintained in a single file in version control. They are typically used for

* (Re-)creating views/procedures/functions/packages/...
* Bulk reference data reinserts

Implementing #209 would probably make this possible with hooks, but I have found it such a useful concept that I think it warrants consideration as a feature.

### Motivating example

I've used this Flyway feature on many projects to manage stored procedures and functions and am really missing it now that I am using Graphile Migrate.

I've tried replicating the functionality with the `afterCurrent` and `afterAllMigrations` hooks, but:

* In `watch` mode, I have to go and make a dummy edit to `current.sql` in order to pick up the changes to my "repeatable" migrations.
* In `migrate` mode, the "repeatable" migrations don't run at all because the migrations have all been applied already.

### Supporting development

I [tick all that apply]:

(I'd love to implement this myself but realistically don't think I would have time, but am happy to assist otherwise).

- [ ] am interested in building this feature myself
- [x] am interested in collaborating on building this feature
- [x] am willing to help testing this feature before it's released
- [ ] am willing to write a test-driven test suite for this feature (before it exists)
- [ ] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile