Allow force running of --once if current.sql has not changed

Feature description

Often we use tricks like select n and increment the number or change current.sql in another way to get it to run.

It would be handy to have a flag to force run --once if the file hasn't changed.

Motivating example

When testing a data migration, it is often very handy to have an idempotent migration that can get run multiple times as you add/change data in your application.

Typical workflow for me in a data migration involves

Keep the original data I am moving in place (if possible)
Run the migration, check for base functionality, compare against existing
Add more data to test the migration and run it again (rinse and repeat as needed)
Once tested, add code to cleanup the original data, test again, and ship

Though workarounds are simple as described in the feature description, I think it would be a nice add.

Breaking changes

n/a

Supporting development

I [tick all that apply]:

am interested in building this feature myself
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)
[ x ] am a Graphile sponsor ❤️
[ x ] have an active support or consultancy contract with Graphile

I think essentially we just want to disable this code:

migrate/src/commands/watch.ts

Lines 86 to 91 in bb2f8f9

    
           migrationsAreEquivalent = 
        
             currentBodyMinified === previousBodyMinified; 
        
           // 4: if different 
        
           if (!migrationsAreEquivalent) { 
        
             await executeActions(

Which is useful to disable in both watch and watch --once modes.

Maybe --always-different (-a) or --no-ignore-unchanged or --no-require-changes or similar might make sense? --force (-f) also makes sense, but it's a bit less specific and limits our future options, perhaps. A --force-when-unchanged flag that --force automatically enables might make sense... but we'd need some more forces (--force-actions) first.

@jemgillam suggests --force-migration which I think is pretty apt - you're forcing the migration to happen even if no changes occurred.

	migrationsAreEquivalent =
	currentBodyMinified === previousBodyMinified;

	// 4: if different
	if (!migrationsAreEquivalent) {
	await executeActions(