move sidecars definitions into plugin-registry

[benoitf]

Is your task related to a problem? Please describe.

Currently, sidecars used in plug-ins are defined in the https://github.com/che-dockerfiles organisation.
It works pretty well for existing plug-ins but it make stuff complicated for contributing:

• when a new plug-in need to be added into the registry with a custom sidecar: a sidecar repository need to be defined, then the image needs to be built and then we can create a PR to the plug-in registry
• testing a plug-in by updating it's runtime (the sidecar) need to be done with 2 PR: one against the sidecar repository and one against the plug-in registry once the image is built. And image is built only if it gets approval and be merged.

Contributor experience should be smoother allowing to:

• do not request the creation of a repository but simply open a PR against che-plugin-repository
• let people test the plug-in with a custom image before even merging the sidecar definition.

Describe the solution you'd like

Allow to add sidecar definitions in the che-plugin-registry referencing the path where sidecar is defined.

example inside a chePlugin:

sidecar:
  path: "../../sidecars/java11"

and in sidecars/java11 folder:
it could contain Dockerfile, entrypoint.sh, etc

Describe alternatives you've considered

Additional context

Lifecycle might be different between sidecars and plug-ins.

[ericwill]

@benoitf any objections to using a monorepo for sidecar images in quay.io? For example: quay.io/che-sidecar-java:8-0cfbacb becomes quay.io/eclipse/che-plugin-sidecar:java8-0cfbacb

This removes the dependency of needing a new quay.io repo for every sidecar image.

[benoitf]

no objection 👍 (any 'manual' step that can be removed is good (like creating quay.io repository and setting-up permissions)