feat(pubsub): add channel registry pub/sub

lib/excache/pubsub.ex

@@ -0,0 +1,14 @@
+defmodule Excache.PubSub do
+  @moduledoc false
+
+  @registry Excache.PubSub.Registry
+
+  def subscribe(channel), do: Registry.register(@registry, channel, nil)
+  def unsubscribe(channel), do: Registry.unregister(@registry, channel)
+
+  def publish(channel, payload) do
+    subscribers = Registry.lookup(@registry, channel)
+    Enum.each(subscribers, fn {pid, _} -> send(pid, {:pubsub, channel, payload}) end)
+    length(subscribers)
+  end
+end