Make WhatsApp bridge configuration playbook-managed

- following spantaleev transition of the telegram brigde
- adding a validate_config task

roles/matrix-bridge-mautrix-whatsapp/tasks/validate_config.yml

@@ -0,0 +1,11 @@
+---
+
+- name: Fail if required settings not defined
+  fail:
+    msg: >-
+      You need to define a required configuration setting (`{{ item }}`).
+  when: "vars[item] == ''"
+  with_items:
+    - "matrix_mautrix_whatsapp_appservice_token"
+    - "matrix_mautrix_whatsapp_homeserver_token"
+