Add health check endpoint to Keystone's express server

[JedWatson]

This adds a configurable health check endpoint to Keystone's express server, avoiding the need to use a custom admin page to implement your own.

It will respond on /_healthcheck with JSON: { status: 'pass', timestamp: Date.now() }

You can change the path and data with the config.server.healthCheck option (documentation to be added in a separate PR)

This PR also adds a response to the dev server while Keystone is initialising, which strictly shouldn't be necessary since you shouldn't be using the dev server in production, but it seems prudent to cover this there anyway just in case.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 2f85233

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@keystone-next/keystone	Patch
@keystone-next/types	Patch

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[timleslie]

Needs changeset

[JedWatson]

@timleslie you got me while I was writing a changeset 😜

Done now, fixed the type too

[timleslie]

👍

[gautamsi]

Not that you have added something for express server, I hope other express server needs will be re evalutated and open soon.

One of the pressing one is bringing back configureExpress feature