Rehome functions to enable Neutron's Trunk service plugin

Those functions were part of the neutron devstack plugin but we
discussed on the neutron team meeting [1] to move it to the Devstack
repo as it's mature enough now.

[1] https://meetings.opendev.org/meetings/networking/2021/networking.2021-10-05-14.00.log.html#l-156

Change-Id: I35446adad1d8a7fed142d834de20c48b611015a5
diff --git a/lib/neutron_plugins/services/trunk b/lib/neutron_plugins/services/trunk
new file mode 100644
index 0000000..8e0f694
--- /dev/null
+++ b/lib/neutron_plugins/services/trunk
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+function configure_trunk_extension {
+    neutron_service_plugin_class_add "trunk"
+}