Add support for optional attributes in the MetaDataReader as defined in the XML schema for metatype

If you look at the metatype spec, 105.8 shows the XML schema for metatype. We have a parser for it called MetaDataReader, and it basically converts the XML file into a tree of Java objects (MetaData and related). The schema, in several locations, allows you to use optional attributes (scan the schema for <anyAttribute />). Our parser currently does not have a way to read such attributes though, but it should!