The schema document for the Shibboleth metadata extensions is missing. It should probably go in idp-saml-api, since that's where the relevant XMLObject provider interfaces live.
It also needs to be effectively added to the schema builder instance 'shibboleth.SchemaBuilder' in global-system.xml. Looks like that's going to require an instance of net.shibboleth.utilities.java.support.xml.SchemaBuilder from java-support to be injected? And looks like the latter needs to be set up with the ClasspathResolver?