Attach Schema Target Namespace to XmlSchemaAny

While I was working on AVRO-457, I found it was difficult to determine the XML Schema's target namespace when I encountered an XmlSchemaAny. This is needed because the XmlSchemaAny's namespace could be ##targetNamespace (the element must match the target namespace) or ##other (the element must be in any namespace other than the target namespace), so it was difficult to confirm whether the element matched the wildcard.

This change adds a "targetNamespace" field to XmlSchemaAny, and has XmlSchemaBuilder attach XmlSchema.getLogicalTargetNamespace() to it. I believe that is the best fit.

Thanks!
Mike