Add support for health checks

[Kaliumhexacyanoferrat]

As a developer using Kubernetes for hosting, I would like to add health and readiness checks to my application, so I can use them in my configuration.

Example

var healthCheck = Health.ReadyWhen(() => AppStatus.Ready);

Acceptance criteria

• The functionality is implemented in a new module
• Health checks are automatically generated and provided via /healthz
• Readness checks are provided by the app logic and provided via /ready
• Health and readiness checks are not listed in the Open API spec
• The feature is documented on the GenHTTP website
• The feature is covered by acceptance tests