Add responsible-party-is-person constraint

[Rene2mt]

Committer Notes

This PR adds responsible-party-is-person constraint, which checks that responsible-party assemblies with role-id set to certain values (e.g., "system-owner", "authorizing-official", "authorizing-official-poc", "system-poc-management", "system-poc-technical", "system-poc-other", and "information-system-security-officer") must reference a party of type "person".

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[Rene2mt]

Added issue GSA/automate.fedramp.gov#57 for required documentation updates.

[aj-stein-gsa]

Will possibly approve after rebase and merge once documentation for the referenced doc PR is complete.

In the meantime, rebase, please and thank you. 🙏

[Rene2mt]

Hey @aj-stein-gsa once I rebased, some of the validations were no longer passing due to modifications I had made to the ssp-all-INVALID.xml test content, so the latest commit rolled those changes back and created a separate test SSP for this new responsible-party-is-person constraint.