Adjusted information-type/*-impact constraints

[nikitawootten-nist]

Committer Notes

Fixes #1795 by allowing other impact values.

What about #1872

#1872 trialed a broader change, allowing for other impact types falling outside of the CIA triad as well as an @ns attribute for optional constraints.

It is clear this issue requires more care. This PR will be followed up with an OSCAL-DEFINE entry usnistgov/OSCAL-DEFINE#34.

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[aj-stein-nist]

Short, sweet, and to the point.