Spiral: Determine approach for unmapped items in a mapping.

[Compton-US]

Problem Statement

Belongs to #18

[Compton-US]

From https://github.com/usnistgov/OSCAL-DEFINE/pull/26/files#r1232653928:

Clarification: the overall gaps analysis could be as simple as a 'source-gap' and 'target-gap' which could point to an external OSCAL profile with the unmapped controls, OR an embedded profile-like list for each, source and destination respectively (when applicable).

[iMichaela]

NOTE: information also captured in the Spiral 2 document: 2023-08-07.003.md

Create: mapping-collection > gap with the following proposed outline:

gap [0 or 1] :
{
    subject [1]: string,
    A choice of:
        include-controls [1] : { 
            with-child-controls [0 or 1]: token,
            with-ids [0 or 1]: [ … ],
            matching [0 or 1]: [ … ],
        },
        link [1]: {
            href [1]: uri-reference,
            rel [0 or 1]: token,
            media-type [0 or 1]: string,
            resource-fragment [0 or 1]: string,
            text [0 or 1]: markup-line,
        },
    props [0 or 1]: [
        An array of prop objects [1 to ∞] {
            name [1]: token,
            uuid [0 or 1]: uuid,
            ns [0 or 1]: uri,
            value [1]: string,
            class [0 or 1]: token,
            group [0 or 1]: token,
            remarks [0 or 1]: markup-multiline,
        },
    remarks [0 or 1]: markup-multiline,
}
```