readiness and liveness endpoints for service health monitoring

[gilescope]

Re-awakening: paritytech/substrate#1017

Stale PR here: paritytech/substrate#14314

What's the current thoughts on having a readiness endpoint? Without an 'official' endpoint people will create ad-hoc checks rather than all collaborate on making one good check.

(If this is already implemented we can close this, but nothing came up on the search)

[bkchr]

CC @niklasad1