Add XML Catalog support to XML Validator

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
        targetNamespace="urn:switchyard-quickstart:validate-xml:0.1.0" 
        xmlns:orders="urn:switchyard-quickstart:validate-xml:0.1.0">
        <xs:import namespace="urn:common-types" schemaLocation="http://www.example.com/types/common.xsd"/>
        ...
</schema>

<?xml version="1.0" encoding="UTF-8"?>
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"> prefer="system">
  <system systemId="http://www.example.com/types/common.xsd"  uri="/xsd/common.xsd" />
  ...
</catalog>