Support a configurable readiness check endpoint via http

[ShuranZhang]

Expose a new string config ReadinessCheckEndpointinconfig.yaml`.
This endpoint is used for monitoring purposes to verify if all configured listeners are successfully started and running.

• To check the status, you need to query the /debug/health path of this endpoint
• It will return an HTTP 200 OK status if all listeners are up and running.
• If not all listeners are ready, it returns HTTP 503 Service Unavailable.
• If this option is not set, the readiness check HTTP endpoint
  will not be registered and no readiness checks will be available via HTTP.

Example output when I setReadinessCheckEndpoint:localhost:8080 in my local experiment:

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to documentation are included in the PR