Run any step from job response in a separate BuildSection

What does this MR do?

Allows for Gitlab to send arbitrary steps back to the runner for execution. It does it by creating a BuildStage for each step coming from Gitlab. Each one is also wrapped in a BuildSection.

Why was this MR needed?

The first use case for this is automated releases (see epic for issue attached). Gitlab will send a 'release' step which will contain a script to call the Releases API and create a release.

The next step will be to be able to run each step in a separate container with a different image.

Here's an example pipeline run with it: https://gitlab.com/lraykov/cicd-experimentation/pipelines/139520020

Follow up issues

• Respect when and allow_failure in steps coming from Rails

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #6409 (closed)