Handle sort-id better as a special control property during generate/assemble

[fsuits]

Issue description / feature objectives

sort-id is an important property for controls and needs to be available in the control markdown whenever assembly happens without access to that property in a separate document. Currently it is allowed as a specially handled key/value pair in the yaml header, but it should be handled more formally

Caveats / Assumptions

Completion Criteria

Define how to define the sort-id for a control Make sure the generate/assemble tools use it when sorting is required

[vikas-agarwal76]

In future we may need to specify OSCAL defined properties in YAML header, in additon to just sort-id. x-oscal-props: sort-id: 'ac-1" label: "AC-1" xyz: "dfgd"