Add sliding-sync support

This allows people to try out the new Element X clients, which need to
run against the sliding-sync proxy (https://github.com/matrix-org/sliding-sync).

Supersedes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2515

The code is based on the existing PR (#2515), but heavily reworked. Major changes:

- lots of internal refactoring and variable renaming

- fixed self-building to support non-amd64 architectures

- changed to talk to the homeserver locally, over the container network (not
  publicly)

- no more matrix-nginx-proxy support due to complexity (see below)

- no more `matrix_server_fqn_sliding_sync_proxy` in favor of
  `matrix_sliding_sync_hostname` and `matrix_sliding_sync_path_prefix`

- runs on `matrix.DOMAIN/sliding-sync` by default, so it can tried
  easily without having to create new DNS records

group_vars/matrix_servers

@@ -330,6 +330,8 @@ devture_systemd_service_manager_services_list_auto: |
     +
     ([{'name': 'matrix-registration.service', 'priority': 4000, 'groups': ['matrix', 'registration']}] if matrix_registration_enabled else [])
     +
+    ([{'name': 'matrix-sliding-sync.service', 'priority': 4000, 'groups': ['matrix', 'sliding-sync']}] if matrix_sliding_sync_enabled else [])
+    +
     ([{'name': 'matrix-sygnal.service', 'priority': 800, 'groups': ['matrix', 'sygnal']}] if matrix_sygnal_enabled else [])
     +
     ([{'name': 'matrix-goofys.service', 'priority': 800, 'groups': ['matrix', 'goofys']}] if matrix_s3_media_store_enabled else [])
@@ -2612,6 +2614,8 @@ matrix_nginx_proxy_systemd_wanted_services_list: |
     +
     (['matrix-jitsi.service'] if matrix_jitsi_enabled else [])
     +
+    (['matrix-sliding-sync-proxy.service'] if matrix_sliding_sync_enabled else [])
+    +
     (['matrix-bot-go-neb.service'] if matrix_bot_go_neb_enabled else [])
     +
     ([etherpad_identifier + '.service'] if etherpad_enabled else [])
@@ -2745,6 +2749,12 @@ devture_postgres_managed_databases_auto: |
       'password': matrix_dendrite_database_password,
     }] if (matrix_dendrite_enabled and matrix_dendrite_database_hostname == devture_postgres_connection_hostname) else [])
     +
+    ([{
+      'name': matrix_sliding_sync_database_name,
+      'username': matrix_sliding_sync_database_username,
+      'password': matrix_sliding_sync_database_password,
+    }] if (matrix_sliding_sync_enabled) else [])
+    +
     ([{
       'name': matrix_ma1sd_database_name,
       'username': matrix_ma1sd_database_username,
@@ -3639,6 +3649,63 @@ matrix_registration_database_password: "{{ '%s' | format(matrix_homeserver_gener
 ######################################################################
 
 
+
+######################################################################
+#
+# matrix-sliding-sync
+#
+######################################################################
+
+# We don't enable the sliding sync proxy by default.
+matrix_sliding_sync_enabled: false
+
+matrix_sliding_sync_scheme: "{{ 'https' if matrix_playbook_ssl_enabled else 'http' }}"
+
+matrix_sliding_sync_hostname: "{{ matrix_server_fqn_matrix }}"
+
+matrix_sliding_sync_path_prefix: /sliding-sync
+
+matrix_sliding_sync_container_image_self_build: "{{ matrix_architecture not in ['amd64'] }}"
+
+matrix_sliding_sync_container_additional_networks: |
+  {{
+    (
+      ([matrix_playbook_reverse_proxyable_services_additional_network] if matrix_playbook_reverse_proxyable_services_additional_network else [])
+      +
+      ([matrix_nginx_proxy_container_network] if matrix_nginx_proxy_enabled and matrix_nginx_proxy_container_network != matrix_sliding_sync_container_network else [])
+      +
+      ([devture_postgres_container_network] if devture_postgres_enabled and devture_postgres_container_network != matrix_sliding_sync_container_network else [])
+    ) | unique
+  }}
+
+matrix_sliding_sync_container_labels_traefik_enabled: "{{ matrix_playbook_reverse_proxy_type in ['playbook-managed-traefik', 'other-traefik-container'] }}"
+matrix_sliding_sync_container_labels_traefik_docker_network: "{{ matrix_playbook_reverse_proxyable_services_additional_network }}"
+matrix_sliding_sync_container_labels_traefik_entrypoints: "{{ devture_traefik_entrypoint_primary }}"
+matrix_sliding_sync_container_labels_traefik_tls_certResolver: "{{ devture_traefik_certResolver_primary }}"
+
+matrix_sliding_sync_systemd_required_services_list: |
+  {{
+    ['docker.service']
+    +
+    ['matrix-' + matrix_homeserver_implementation + '.service']
+    +
+    ([devture_postgres_identifier ~ '.service'] if devture_postgres_enabled else [])
+    +
+    (['matrix-nginx-proxy.service'] if matrix_nginx_proxy_enabled else [])
+  }}
+
+matrix_sliding_sync_environment_variable_syncv3_secret: "{{ '%s' | format(matrix_homeserver_generic_secret_key) | password_hash('sha512', 'ss.secret', rounds=655555) | to_uuid }}"
+
+matrix_sliding_sync_database_hostname: "{{ devture_postgres_connection_hostname if devture_postgres_enabled else '' }}"
+matrix_sliding_sync_database_password: "{{ '%s' | format(matrix_homeserver_generic_secret_key) | password_hash('sha512', 'ss.db', rounds=655555) | to_uuid }}"
+
+######################################################################
+#
+# /matrix-sliding-sync
+#
+######################################################################
+
+
 ######################################################################
 #
 # matrix-dendrite