Readiness api endpoint

### Feature description

worker API that can determine whether worker is up-and-running

### Motivating example

when kubernetes is running a pod it has several probes to determine its status (startup, readines, liveness)
while I could `touch` a file on an event handler this would require ephemeral storage on a read-only filesystem.

A useful pro feature could be to allow for an endpoint `/readiness` which can respond with a 200 when worker is up and running.

### Breaking changes

None

### Supporting development

I [tick all that apply]:

- [ ] 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)
- [x] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile