Put HTTPSignatures, WebFinger and ActivityStream alongside with ActivityPub

lib/federation/activity_stream/convertible.ex

@@ -0,0 +1,10 @@
+defprotocol Mobilizon.Federation.ActivityStream.Convertible do
+  @moduledoc """
+  Convertible protocol.
+  """
+
+  @type activity_streams :: map
+
+  @spec model_to_as(t) :: activity_streams
+  def model_to_as(convertible)
+end