WIP - Feature issue 564 - FedRAMP Extensions Registry

[Rene2mt]

Committer Notes

DO NOT MERGE. This PR is a Work In Progress (WIP).

This PR proposes a solution for issue #564.

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[aj-stein-gsa]

So thanks so so much for writing this ADR. It actually served to clear up some things in my head around the gap and the solution it ought to fill.

I first have a high-level question, do we need a registry as a formalized product we maintain, or do we need better documentation point back to the applicable source (i.e. high-fidelity machine-readable info in Metaschema constraint definitions)?