feat: support loading Elixir modules for auth

Allow to specify an Elixir module name in `auth_method`.

If the referenced module, `M`,  cannot be loaded as `ejabberd_auth_M`,
try to load it as `Elixir.M`.

lib/ejabberd_auth_example.ex

@@ -0,0 +1,39 @@
+defmodule ModAuthExample do
+  @moduledoc """
+
+  This is a dummy auth module to demonstrate the usage of Elixir to
+  create Ejabberd Auth modules.
+
+  """
+  import Ejabberd.Logger
+  @behaviour :ejabberd_auth
+
+  @impl true
+  def start(host) do
+    info("Using mod_auth_example to authenticate #{host} users")
+    nil
+  end
+
+  @impl true
+  def stop(host) do
+    info("Stop using mod_auth_example to authenticate #{host} users")
+    nil
+  end
+
+  @impl true
+  def check_password("alice", _authz_id, _host, "secret"), do: {:nocache, true}
+  def check_password(_username, _authz_id, _host, _secret), do: {:nocache, false}
+
+  @impl true
+  def user_exists("alice", _host), do: {:nocache, true}
+  def user_exists(_username, _host), do: {:nocache, false}
+
+  @impl true
+  def plain_password_required(_binary), do: true
+
+  @impl true
+  def store_type(_host), do: :external
+
+  @impl true
+  def use_cache(_host), do: false
+end