config file to manage current migration order

### Feature description

Add the option of adding a config file that goes with sql files that specifies the order of execution of a current migration, as an alternative to numbered sql files.

### Motivating example

adding files not at the end of a migration file list or reordering files creates lots of problems for merging into version control, and makes hard to look at the history of a piece of code.

example:

existing current migration:
001-A.sql
002-B.sql
003-C.sql

... add file at position 2

001-A.sql
002-NEW.sql
003-B.sql
004-C.sql

now files B and C will be hard to merge, and may or may not have file histories in version control.

proposed change:

existing current migration:

A.sql
B.sql
C.sql
config-file

... add file at position 2

A.sql
B.sql
C.sql
NEW.sql
updated-config-file

### Supporting development

I [tick all that apply]:

- [x] 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
- [x] 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