OSCAL and FedRAMP OSCAL extensions external constraints

[david-waltermire]

Committer Notes

This PR provides external constraints for use with the OSCAL-cli.

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.

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• 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?

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