core: fix: compilation error in generated classes when body or response is a generic type
core: config resource should be part of admin group
core - servlet - simple: factorize how toString is implemented in RestxRequest implementations
factory: don't dump whole factory on dependency injection errors
factory: provide similar components when a dependency is not satisfied
factory: give details when factory builder is failing to load factory machines from service loader

Often the problem is due to a rename or removed component, give this hint for the developer.
server: jetty+tomcat - check appbase and webinf locations before starting web server (fixes #26)
server: check server socket can be opened before starting the server

This makes the server start actually fail when it isn't able top open the port, which is easier to diagnose especially during tests.

Add better error reporting for "address already in use" which is one of the most common case during development, and the message is not always clearly understood, especially by beginners.
core: allow to create a RestxMainRouter from RestxMainRouterFactory without a WebServer associated with it

 This allow to use restx in other contexts than web servers.