Support adding responsible role information in the control markdown

[jpower432]

Issue description / feature objectives

When editing the YAML header with trestle generate commands, it would be helpful to be able to allow users to specify responsible-roles information at the implemented requirement and statement level to reduce manual editing of the OSCAL JSON file.

Caveats / Assumptions

• This would apply to the implemented-requirement and statement assemblies only
• There would have to be a (separate) way to author the roles under the metadata in the SSP

Completion Criteria

• A field is added to the YAML header to support adding responsible role details in the control markdown
• The roles that can be chosen (from the roles metadata) are displayed or accessible to ensure the user knows what roles to choose from