Add subsystem parsing of persistence.xml documents in the EE 9 namespace

The schema used for persistence.xml documents in EE 9 is functionally equivalent to what was used in EE 8; it just has a different namespace, schemaLocation and version, plus various documentation text or type name changes.

So the existing PersistenceUnitXmlParser could easily parse these if it were made aware of how to recognize the schema version. Doing this is necessary to allow the EE 9 variant of WildFly to parse native EE 9 descriptors. Doing this in the EE 8 variant of the server does not harm.

Note this is not about producing objects that use EE 9 APIs (i.e. jakarta.* packages instead of javax.*.)

See also JBMETA-419.