New job_key_mode - abort_replace

### Feature description

(I am evaluating Graphile worker to see if it could fit the use case I have in mind)

I would like a new `job_key_mode` mode perhaps called `abort_replace` which would trigger abort on the locked task with the matching `job_key`

### Motivating example

I have a job which takes a while to complete and relies on data. If a new job has been submitted which has the same `jobKey` then the locked task is running with stale data and can be aborted since the result of the task won't be needed

### 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