Create xml instruction to add a Maven dependency to a project

In order to avoid unnecessary rebuilds during plugin installation, some plugins wish to install their (non-essentials-)dependencies during the second stage of the installation process, i.e. instruction-based rather then descriptor-based. Examples are the projects and the lucene index exporter features. In order to avoid duplicate code and unnecessarily complex custom instructions, this should be facilitated through a new instruction, defined through the plugin's instructions.xml file.

Implementation of this instruction depends on ESSENTIALS-1113.