Stack build and downstream tests

[fmauch]

Since #2 became quite a discussion, I think it does make sense to add a separate issue for discussion so we can focus on the actual PR(s) a bit more.

I think, the things discussed should be split into the following PRs:

• Add TARGET_WORKSPACE to the "normal" ici workflow (Add target_workspace to ICI workflow #3)
• Add DOWNSTREAM_WORKSPACE to the "normal" ici workflow (Add possibility to specify a DOWNSTREAM_WORKSPACE with ici #2)
• Add the complete stack build to this repo (Build full stack #5)
• Remove the complete stack build from control.ros.org (Could be part of [CI] Use ros2_control_ci control.ros.org#244)
• Remove the target_ws workflow from this repo since it is then no longer needed Remove 2nd industrial_ci wf as it is included in the other one now #6
• Add DOWNSTREAM_WORKSPACE definitions where we want them (still to be discussed)