Implementation of a build flow extension point

The new class BuildFlowPluginExtension exposes our external resource
manager to the build flow plugin so that it can be accessed from
within a build flow script like this:

  def manager = extension.'externalresource-dispatcher'
  manager.lock(...)

This allows us easy access to the manager without any imports
littering our build flow script

The code is a contribution from Ericsson AB (http://www.ericsson.com/) and has been reviewed internally by Marco Miller (https://github.com/marco-miller)