Add contributions export in Nuxeo Package

Goal is to have a JSON file containing Nuxeo Package contributions inside the package.

For now, extraction is done by studio-maven-plugin but has to be part of the platform, and moved to nuxeo-apidoc-core module.

The issue is tied to the packaging as a Maven plugin that create a "dependency" circle if we need to use and release the plugin from inside the same build.

However, we could make this new component work in 2 modes:

Standalone jar file

• Called by introspection inside the main Nuxeo build

Work as a maven plugin (once released)

• Called like that from external addons using a Nuxeo CLI generated pom
  This basically means that:

During the build we use the "jar version" from within Nuxeo
When doing an addon, we use the Maven plugin released with previous Nuxeo version