enable 3rd party plugins to propagate events through the cluster

[ballinette]

Idea: expose to plugins a method to trigger an event that will be propagated to other cluster nodes.

example :

class MyPlugin {
  // Hook configuration:
  constructor () {
    this.hooks = {
      'plugin-my-plugin:my-custom-event': 'myFunction'
    };
  }
  myFunction(payload) {
    this.doSomething(payload);
  }

  
  // broadcast an event to other nodes:
  myCustomMethod() {
    // (....)
    // if the plugin-cluster is not enabled => the `broadcast` method will do nothing
    // if it is enabled, it will encapsulate the event, send it to other cluster nodes,
    //                         and then in each node, trigger a `plugin-my-plugin:my-custom-event` hook:
    context.accessors.broadcast('my-custom-event', payload);
  }

see also comments here: kuzzleio/documentation#339 (comment)