Implement and test 'ANY' feature in XSD and JSON Schema

[wendellpiez]

User Story:

See usnistgov/OSCAL#792.

Goals:

Support ANY feature as "any other" (namespace) in XML or "additionalProperties" : true in JSON Schema.

Note: Deciding where to use ANY in the OSCAL models is not a goal of this Issue and may bear separate discussion.

Dependencies:

None at this time. Could be addressed together with the EMPTY object (assembly) type. Cf #49, also #33 and #66.

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.