MULE-8028 - adding support for mule client in http connector

This is not part of the PR but I didn't see it before.
Configuration element is for things that affect mule core. Attribute useHttpTransportByDefault should not go here in my opinion. A system property would be better in this case.
Besides that, the name can confuse users thinking that setting this in true/false will change everything related to HTTP, when it only affects the uses from muleclient