Removing hard dependency from restx-specs-test to restx-server-jetty, and demonstrating issue #29

Relying on a new WebServerSupplierFactory to inject WebServerSupplier from BOM.
The restx-specs-test rules will then rely either on @Procuced WebServerSupplier (if no WebServerSupplier is passed as a constructor arg to the @Rule)

Important note : this introduced a backward incompat in restx-spects-test @Rule constructors, too much tied to the restx-server-jetty implementation, which was removed.
To fix this, you will either have :
- To use simple default constructor which will rely on default web server config available in classpath
- To provide a WebServerSupplier *by hand* if you want to override default web server config

Another backward compat is `ServerSuppliers` utility class doesn't exist anymore : `WebServerSupplier` can be retrieved directly from the restx-server-* utilities (for instance, `JettyWebServer.jettyWebServerSupplier()`)