Extend to support multiple designated (valid) root nodes

[wendellpiez]

User Story:

In support of usnistgov/OSCAL#590 and usnistgov/OSCAL#595, we need the capability to define any assembly as a valid root (element or object) in a metaschema. In addition to being potentially useful, this will remedy a discrepancy in the JSON and XML expressions of a given metaschema.

Goals:

Currently, an XSD generated from a metaschema can be used to validate a document with any root element declared in the metaschema. Contrarily, the JSON Schema permits a document rooted only at the nominal root (e.g., catalog for the catalog model).

This is either to tight (JSON) or too loose (XML). A Goldilocks solution would permit a metaschema author to designate which assemblies (if not also fields) can serve as valid roots, and the XML and JSON validations should correspond.

Dependencies:

This work should be done with unit tests.

Acceptance Criteria

• All website and readme documentation affected by the changes in this issue have been updated. Changes to the website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.