Upgrade to JAX-RS 2 (JSR339)

JAX-RS 1 (JSR311) is pretty old now and requiring it is starting to be an issue on some application servers (Tomcat 9, WebSphere 12) and is blocking the use of some dependencies (Keycloak for example but the list is going to grow fast if not already very long) and extensions that would directly or indirectly require the version 2.

Unfortunately it's not an easy upgrade since Restlet is not fully working with JAX-RS 2 (great retro compatibility on JAX-RS side...) and seems to be pretty much dead so I doubt we can expect a JAX-RS 2 based version of Restlet anytime soon...