Add configuration for gitlab-runner binary

What does this Merge Request do?

To be able to build a custom gitlab-runner binary and manage it using the GDK, I want to be able to specify the path of the binary. This MR adds a new configuration option, used like:

runner:
  bin: ../gitlab-runner/out/binaries/gitlab-runner-darwin-amd64

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.