Add (native) Traefik support to synapse-admin

Previously, it had to go through matrix-nginx-proxy.
It's exposed to Traefik directly via container labels now.

roles/custom/matrix-synapse-admin/tasks/validate_config.yml

@@ -10,3 +10,24 @@
     - {'old': 'matrix_synapse_admin_docker_repo', 'new': 'matrix_synapse_admin_container_self_build_repo'}
     - {'old': 'matrix_synapse_admin_container_self_build', 'new': 'matrix_synapse_admin_container_image_self_build'}
     - {'old': 'matrix_synapse_admin_container_self_build_repo', 'new': 'matrix_synapse_admin_container_image_self_build_repo'}
+
+- when: matrix_synapse_admin_container_labels_traefik_enabled | bool
+  block:
+    - name: Fail if required matrix-synapse-admin Traefik settings not defined
+      ansible.builtin.fail:
+        msg: >-
+          You need to define a required configuration setting (`{{ item }}`).
+      when: "vars[item] == ''"
+      with_items:
+        - matrix_synapse_admin_container_labels_traefik_hostname
+        - matrix_synapse_admin_container_labels_traefik_path_prefix
+
+    # We ensure it doesn't end with a slash, because we handle both (slash and no-slash).
+    # Knowing that `matrix_synapse_admin_container_labels_traefik_path_prefix` does not end with a slash
+    # ensures we know how to set these routes up without having to do "does it end with a slash" checks elsewhere.
+    - name: Fail if matrix_synapse_admin_container_labels_traefik_path_prefix ends with a slash
+      ansible.builtin.fail:
+        msg: >-
+          matrix_synapse_admin_container_labels_traefik_path_prefix (`{{ matrix_synapse_admin_container_labels_traefik_path_prefix }}`) must not end with a slash.
+          Example: `/synapse-admin`.
+      when: "matrix_synapse_admin_container_labels_traefik_path_prefix[-1] == '/'"