Add Health checks

[vordimous]

Describe the desired outcome from the user's perspective
As a DevOps maintainer optimizing Zilla deployments, I would expect Zilla to expose the status of its internal connections and services. If Zilla can determine if a downstream service is unreachable or degraded, It could provide an API to report the status. Any internal systems that can be in a Green, Yellow, or Red state would be helpful in reporting that status to an orchestration layer.

Acceptance criteria

• acceptance criteria 1
• acceptance criteria 2

Additional context

• The closet thing to an HTTP standard spec I could find Health Check Response Format for HTTP APIs
• K8s liveliness/readiness/startup probes
• good blog post A guide to API health check