Support optional helper executable in custom executor

What does this MR do?

The docker executer uses a special helper image with several tools pre-installed to run all the predefined build steps. The custom executor does not yet have any distinction between those predefined steps and user defined steps. This merge request adds an additional executable to the custom executor that is used to run those predefined steps.

Why was this MR needed?

This MR allows to more closely match the behaviour of the docker executor using the custom executor. All the predefined steps can be run using a different image without re-implementing the selection logic between predefined and user defined in the run executable. I'm currently replacing docker-machine with some script logic handling VM and a podman remote connection.

What's the best way to test this MR?

• Specify a helper executable, either with --custom-helper-exec on the command line or with helper_exec in the config, that runs in a distinctive different environment.

What are the relevant issue numbers?