OSCAL CLI and External Constraints Improvements

[david-waltermire]

Committer Notes

This feature branch provides external constraints for use with the OSCAL-cli and CI/CD improvements.

Two files are provided:

• fedramp-external-constraints.xml - contains FedRAMP-specific OSCAL extensions expressed using the Metaschema external constraints format. These constraints are intended to check for content required by FedRAMP as defined in the FedRAMP OSCAL guides.
• oscal-external-constraints.xml - contains additional OSCAL constraints that check core OSCAL expectations that are not currently enforced in the OSCAL Metaschemas. This includes checking many cross-document relationships. Once these have been tested, these constraints will be contributed to the NIST OSCAL repo.

All Submissions:

• [x] Have you selected the correct base branch per Contributing guidance?
• [x] Have you set "Allow edits and access to secrets by maintainers"?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Have you squashed any non-relevant commits and commit messages? [instructions]
• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• ~If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated?~
• ~If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?~

By submitting a pull request, you are agreeing to provide this contribution under the CC0 1.0 Universal public domain dedication.

[wandmagic]

i think this is ready to be converted to a real PR and review, so we can start making PR's into develop instead of this feature branch