Add support for Gradle Apt Plugin

I already have another package that uses annotation processing (immutables), and it doesn't play nicely with the documented Gradle solution for adding annotation processing.

immutables plugs in directly to the Gradle Apt Plugin (https://github.com/tbroyer/gradle-apt-plugin)

It would be nice if the CodeGen worked similarly for vertx.. I have it working with Warnings only via:

dependencies {
compileOnly "org.immutables:value:2.4.4:annotations"
apt group: 'org.immutables', name: 'value', version: '2.4.4'

compileOnly group: 'io.vertx', name: 'vertx-codegen', version: vertxVersion
apt group: 'io.vertx', name: 'vertx-codegen', version: vertxVersion
apt group: 'io.vertx', name: 'vertx-docgen', version: vertxVersion
apt group: 'io.vertx', name:'vertx-service-proxy', version:vertxVersion, classifier: 'processor'
}

This generates both sets of classes but my gradle output is littered with warnings:

warning: No SupportedSourceVersion annotation found on io.vertx.serviceproxy.ServiceProxyProcessor, returning RELEASE_6.
warning: Supported source version 'RELEASE_6' from annotation processor 'io.vertx.serviceproxy.ServiceProxyProcessor' less than -source '1.8'

I have java code and source compatibility set to 1.8 but can't figure out how to feed this in to avoid the warning. Hopefully this is just a documentation issue, would be helpful to add this as an option to the documentation for anyone else using the apt plugin.

Iniziare con la soluzione documentata per l’elaborazione delle annotazioni di Gradle e confrontarla con la configurazione di Gradle Apt Plugin mostrata nell’issue. Documentare come configurare la generazione del codice Vert.x con il plugin e la compatibilità del codice sorgente con Java 8, in modo da risolvere gli avvisi del processore segnalati; il lavoro è completato quando la documentazione pertinente include una configurazione equivalente funzionante.

Scritto dal modello di indicizzazione a partire dal testo della issue.