Add appservice-double-puppet service for better bridge double-puppeting

Bridges will be switched to this new method in future patches.

roles/custom/matrix-appservice-double-puppet/tasks/validate_config.yml

@@ -0,0 +1,10 @@
+---
+- name: Fail if required matrix-appservice-double-puppet settings not defined
+  ansible.builtin.fail:
+    msg: >
+      You need to define a required configuration setting (`{{ item.name }}`).
+  when: "item.when | bool and vars[item.name] == ''"
+  with_items:
+    - {'name': 'matrix_appservice_double_puppet_registration_as_token', when: true}
+    - {'name': 'matrix_appservice_double_puppet_registration_as_token', when: true}
+    - {'name': 'matrix_appservice_double_puppet_registration_sender_localpart', when: true}