Support for post-transform of workflows

[kenmuse]

When migrating a workflow, it's often necessary to apply additional logic to the completed workflow or job. For example, there's no way to:

• Replace a pattern of steps with an optimal single step
• Add a new setup task to the start of each job (esp when using self-hosted runners)
• Cleanup environment variables that represent secrets (esp. in CircleCI)
• Modifying the generated triggers for the workflow

Currently, the transform method (and related) only allow you to modify specific parts of the workflow as they are encountered. Having a transformation method which runs at the end of the process could enable these use-cases.

The proposed functionality would be a transformation method that receives an Hash or object that represents the final generated workflow. Ideally, it would have a second input representing the source that was transformed. If this transform is present, it would return a Hash that represents the final workflow that will be rendered. If multiple transforms are presented, they would be chained together (or rejected as unsupported).