Introduce Maven Enforcer Extension

With MENFORCER-142 it was made possible to execute additional rules in case you couldn't touch the pom.
This was done via a commandline property, but lacked the support for finegrained configuration as you can do with a plugin.

The Maven Enforcer Extension does support this. It'll make use of the Maven extension mechanism, hooking into the build lifecycle.
This extension will read its configuration from a predefined location, and will add these as new execution blocks to the buildplan.