Allow OSCAL metadata to be edited through the trestle `author` commands

[jpower432]

Issue description / feature objectives

The objective would be to add a separate file for managing OSCAL metadata that can be used in conjunction with the control markdown.

Caveats / Assumptions

• The file to be edited would be YAML
• The information would be available in the control markdown as read-only (directly or via link)

Completion Criteria

• Initially support populating roles, parties, and responsible-parties