Introduce module to format WS messages how the server expects them

This is what the dashboard server expects. At the moment this is here
to capture what we discovered about the Node agent which uses Primus.

lib/poa_agent/format/primus_emitter.ex

@@ -0,0 +1,7 @@
+defmodule POAAgent.Format.PrimusEmitter do
+  @moduledoc false
+
+  def write(event: e, data: d) do
+    %{emit: [e, d]}
+  end
+end