Setup OSCAL for usnistgov/metaschema#240 testing

Per request from @wendellpiez for debugging, I am going to set this up
to add to a test submodule and see emmitted JSON Schema result and
validation against catalogs in usnistgov/oscal-content examples.

src/metaschema/oscal_catalog_metaschema.xml

@@ -139,7 +139,7 @@
                   <!--<any/>-->
             </model>
             <constraint>
-                  <allowed-values target="prop[has-oscal-namespace('http://csrc.nist.gov/ns/oscal')]/@name">
+                  <allowed-values target="prop[has-oscal-namespace('http://csrc.nist.gov/ns/oscal')]/@name" allow-other="yes">
             &allowed-values-control-group-property-name;
                   </allowed-values>
                   <allowed-values target="part[has-oscal-namespace('http://csrc.nist.gov/ns/oscal')]/@name">