Add a testcase to demonstrate HTTP/2 Support

CXF 3.5 will provide HTTP/2 Transport support: https://issues.apache.org/jira/browse/CXF-8605

Since 4.0.1, CXF added the HTTP/2 support for client side:https://github.com/apache/cxf/commit/9b36a4bc996615e0ed02795c74167586a2bb11df

We need to look at if we need to 

• change integration code to support this feature 
• tests or examples to demonstrate this new feature 
• documentation update