Oscal issue678 milestone3 enhancements

[wendellpiez]

Committer Notes

This PR addresses functionality gaps in Metaschema for OSCAL Milestone 3 as described in usnistgov/OSCAL#678.

The second gap identified had already been addressed by PR #13 and will also be in next-gen Metaschema.

The first requirement is support for an actual (lexical) JSON data type for a "uuid". Note: currently, we do not validate the uniqueness of UUID values or indeed (always) ID values in the generated schemas. This support is so far only for the lexical form of a string (flag or field value) expected to be a UUID.

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• Have you followed the guidelines in our Contributing document?
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Changes to Core Features:

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• Have you included examples of how to use your new feature(s)?
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[david-waltermire]

LGTM!