Readiness api endpoint

[purge]

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
• am interested in collaborating on building this feature
• 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 ❤️
• have an active support or consultancy contract with Graphile

[benjie]

It's possible to build a system for this based on the eventing system #155 - I've done so for clients before. And yes, this is one of the planned features for Pro; Worker itself will never come with a bundled HTTP server.