Add matrix-user-creator role - automatic user account creation support

We no longer ask users to create Matrix user accounts for these bots:

- Postmoogle
- Honoroit
- Reminder Bot

Other bots and services (matrix-registration-bot, maubot, mjolnir,
Dimension, etc.) require an Access Token to run (not a password),
so this new role doesn't help for them.

It does help for the above bots though, and for defining your own
"initial user accounts" in the `matrix_user_creator_users_additional`
variable.

roles/matrix-user-creator/tasks/util/ensure_user_registered_dendrite.yml

@@ -0,0 +1,17 @@
+---
+
+- name: Ensure Dendrite user registered - {{ user.username | quote }}
+  ansible.builtin.command:
+    cmd: |-
+      {{ matrix_host_command_docker }} exec matrix-dendrite
+      create-account
+      -config /data/dendrite.yaml
+      -username {{ user.username | quote }}
+      -password {{ user.initial_password | quote }}
+      {% if user.initial_type == 'admin' %}
+        -admin
+      {% endif %}
+      -url http://localhost:{{ matrix_dendrite_http_bind_port }}
+  register: matrix_dendrite_register_user_result
+  changed_when: matrix_dendrite_register_user_result.rc == 0 and 'Desired user ID is already taken' not in matrix_dendrite_register_user_result.stderr
+  failed_when: matrix_dendrite_register_user_result.rc != 0 and 'Desired user ID is already taken' not in matrix_dendrite_register_user_result.stderr