Automatically expose /_synapse/admin for Dendrite when synapse-admin is enabled

This is what we do for Synapse as well.

group_vars/matrix_servers

@@ -4821,6 +4821,8 @@ matrix_dendrite_container_labels_traefik_docker_network: "{{ matrix_playbook_rev
 matrix_dendrite_container_labels_traefik_entrypoints: "{{ devture_traefik_entrypoint_primary }}"
 matrix_dendrite_container_labels_traefik_tls_certResolver: "{{ devture_traefik_certResolver_primary }}"
 
+matrix_dendrite_container_labels_client_synapse_admin_api_enabled: "{{ matrix_synapse_admin_enabled }}"
+
 matrix_dendrite_container_labels_client_root_redirection_enabled: "{{ matrix_dendrite_container_labels_client_root_redirection_url != '' }}"
 matrix_dendrite_container_labels_client_root_redirection_url: "{{ (('https://' if matrix_playbook_ssl_enabled else 'http://') + matrix_server_fqn_element) if matrix_client_element_enabled else '' }}"