Add CI job state for waiting for external action

Proposal

Add a new job status, waiting_for_callback, which will represent a CI job that is currently being executed by a service external to GitLab.

When a job that is external (CI YAML syntax for external jobs (#415737)) is triggered, instead of transitioning to the pending state and being picked by a runner, it will enter waiting_for_callback and remain there until the external service provides a valid result via API (Add API endpoint for setting the status of exte... (#415738)).

Once in the waiting_for_callback state, an external job does nothing until either:

• A success or failure result is received via API
• The job or pipeline timeout is reached
• The pipeline is cancelled

Definition of done

• waiting_for_callback is added to the list of valid CI build and pipeline states. !134605 (merged)
• External jobs model created. !134938 (closed)
• Timeouts are enforced for external jobs. !134938 (closed)
• [-] A job in the waiting_for_callback state can be viewed from the UI. #417141
• Import/export supported for external jobs. !135678 (closed)