Add ExtensionParsingContext convenience method for registering a set of schemas

ExtensionParsingContext provides an abstraction over an XMLMapper for registering subsystem parsers. Therefore it makes sense to add an equivalent to https://github.com/jbossas/staxmapper/blob/1.4.0.Final/src/main/java/org/jboss/staxmapper/XMLMapper.java#L69
This further eliminates boilerplate code within Extension implementations.