Test upstream che-theia PR on downstream CRW

We should have a check that automatically check if a PR against che-theia is working for codeready-workspaces-theia

So it would result in a fail-fast system.

Requirements:

• crw-theia sources need to be public (not just in pkgs.devel), so this depends on CRW-1275 (inserting crw-theia sources into crw-images midsteam repo)

• need to have a bleeding-edge branch (main? head? crw-2.7-rhel-8?) that's ahead of stable branch (crw-2.6-rhel-8) and is kept current w/ changes needed to make 2.6 happen
  • we could switch back to the old workflow of 'do everything in head until we're stable / code freeze, then cut the 2.x branch from head'
  • new casc jenkins will make it easier to have stable and bleeding edge jobs running in parallel, but we'll need to do some refactoring to ensure we can map a "head" branch to the correct pkgs.devel branch, if it's no longer the simple 1:1 mapping of
    • midstream crw-theia crw-2.6-rhel-8 to
    • downstream pkgs.devel crw-2.6-rhel-8

• need GH action(s) in crw-theia and/or crw-images repos, that can be triggered on changes to che-theia
• need crw-bot token added to che-theia repo, so that che-theia GH action can invoke actions via API across organizations

Additional Reqs to clarify:

• need to have future-branch 2.7 already created in pkgs.devel in order to be able to use brew for nightlies, or
• need a brew-bypass mechanism in Jenkins job get-sources rhpkg container build to skip brew and build in Jenkins directly
• can CPaaS handle up-to-mid validation (prevent commits until PR passes) and then orchestrate building nightly scratch builds / containers?