Trigger a re-usable workflow

[zarthross]

The current WorkflowJob class emphasizes 'steps' but there is another way to create job by using a reusable workflow.

In order to add support for reusable workflows in our current WorkflowJob type should we.

1. Add uses, with and secrets to the current WorkflowJob
2. Split WorkflowJob into 2 sub types, WorkflowStepJob (or something) and WorkflowReusableJob with the parent of WorkflowJob.

The cons to approach 1 is uses, with, and secrets are ONLY for reusable jobs and would introduce the ability to construct an invalid job.

The cons to approach 2 are that its clearly binary breaking. But this MIGHT be worth it given the issues with approach 1.

Happy to submit a PR to address this, but would like to know which approach to use before i begin.