Initial work on Traefik support

This gets us started on adding a Traefik role and hooking Traefik:

- directly to services which support Traefik - we only have a few of
  these right now, but the list will grow

- to matrix-nginx-proxy for most services that integrate with
  matrix-nginx-proxy right now

Traefik usage should be disabled by default for now and nothing should
change for people just yet.

Enabling these experiments requires additional configuration like this:

```yaml
devture_traefik_ssl_email_address: '.....'

matrix_playbook_traefik_role_enabled: true
matrix_playbook_traefik_labels_enabled: true

matrix_ssl_retrieval_method: none

matrix_nginx_proxy_https_enabled: false

matrix_nginx_proxy_container_http_host_bind_port: ''
matrix_nginx_proxy_container_federation_host_bind_port: ''

matrix_nginx_proxy_trust_forwarded_proto: true

matrix_nginx_proxy_x_forwarded_for: '$proxy_add_x_forwarded_for'

matrix_coturn_enabled: false
```

What currently works is:
reverse-proxying for all nginx-proxy based services **except** for the Matrix homeserver
(both Client-Server an Federation traffic for the homeserver don't work yet)

roles/custom/matrix-nginx-proxy/tasks/setup_nginx_proxy.yml

@@ -22,6 +22,14 @@
     - "{{ matrix_nginx_proxy_data_path }}"
     - "{{ matrix_nginx_proxy_confd_path }}"
 
+- name: Ensure Matrix nginx-proxy labels file is created
+  ansible.builtin.template:
+    src: "{{ role_path }}/templates/labels.j2"
+    dest: "{{ matrix_nginx_proxy_base_path }}/labels"
+    owner: "{{ matrix_user_username }}"
+    group: "{{ matrix_user_groupname }}"
+    mode: 0640
+
 - name: Ensure Matrix nginx-proxy configured (main config override)
   ansible.builtin.template:
     src: "{{ role_path }}/templates/nginx/nginx.conf.j2"