CORE-898 fixing issue that it is impossible to include extensions on Preconditions in an xsd validated xml changelog

CORE-898 - fixing missing xdd entries to allow extensions on precondions in xml. CORE-898 was wrongly closed, it was not fixed

This should be updated in the xsds accessible from:

http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-2.0.xsd
http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-3.0.xsd