Support adding generic properties during SSP authoring

[jpower432]

Issue description / feature objectives

When a custom YAML header is used with ssp generate, it would be helpful to be able to specify extra properties at the implemented requirement and statement level.

An example use case for this would be adding implementation-status and control-origination properties to meet FedRAMP OSCAL SSP requirements without having to edit the JSON.

Caveats / Assumptions

• This would support generic properties and there would be no special handling per framework (i.e. this will not map to specific OSCAL fields)
• The x-trestle-add-props logic for profile authoring could be leveraged

Completion Criteria

• Similar to the profile markdown, add a comment section that can be uncommented to process custom properties.
• This must support adding namespace information for each property or adding applicable properties under a higher level namespace key in a nested fashion.
• This must support adding properties to implemented requirements and statements (not at the by-component level)

[jpower432]

cc @vikas-agarwal76 @degenaro Please let me know if anything written here does not line up with what was agreed on