Che: Extend plugin meta model to include health check endpoint

[amisevsk]

Is your enhancement related to a problem? Please describe.

One of the steps in workspace startup is waiting for any servers to be ready. It would be convenient if plugin specs could define an endpoint which can be pinged to check for server readiness/health.

Describe the solution you'd like

Add health check endpoints to plugin meta model. Potentially a liveness and readiness definition, parallel to what exists in Kubernetes.

Describe alternatives you've considered

Rely on communicating with a port defined for the plugin.

Additional context

The Che workspace operator needs to be able to check for server readiness before marking a workspace as 'Running', to avoid 503 errors. See PR https://github.com/che-incubator/che-workspace-operator/pull/78 for details.

[ibuziuk]

I believe we have similar issues on Hosted Che when for example theia route is not yet available - https://github.com/redhat-developer/rh-che/issues/1657 / https://github.com/eclipse/che/issues/17542

[ibuziuk]

@ericwill I put P1, but feel free to update the priority based on the team priorities

[amisevsk]

Not sure which area this one actually falls under -- there's a plugin side change somewhere, but the plugin spec changes would mostly be handled by infra/plugin broker. @ibuziuk @ericwill WDYT about:

• Step 1: add to plugin meta.yaml, etc. spec, and implement handling in wsmaster
• Step 2: try to add support to actual plugins (machine-exec, theia, etc.)

[sleshchenko]

Is it something that should be addressed in Devfile 2.x format? @l0rd @davidfestal WDYT?