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

roles/custom/matrix-sliding-sync/templates/systemd/matrix-sliding-sync.service.j2

@@ -0,0 +1,49 @@
+#jinja2: lstrip_blocks: "True"
+[Unit]
+Description=matrix-sliding-sync
+{% for service in matrix_sliding_sync_systemd_required_services_list %}
+Requires={{ service }}
+After={{ service }}
+{% endfor %}
+{% for service in matrix_sliding_sync_systemd_wanted_services_list %}
+Wants={{ service }}
+{% endfor %}
+DefaultDependencies=no
+
+[Service]
+Type=simple
+Environment="HOME={{ devture_systemd_docker_base_systemd_unit_home_path }}"
+ExecStartPre=-{{ devture_systemd_docker_base_host_command_sh }} -c '{{ devture_systemd_docker_base_host_command_docker }} kill matrix-sliding-sync 2>/dev/null || true'
+ExecStartPre=-{{ devture_systemd_docker_base_host_command_sh }} -c '{{ devture_systemd_docker_base_host_command_docker }} rm matrix-sliding-sync 2>/dev/null || true'
+
+ExecStartPre={{ devture_systemd_docker_base_host_command_docker }} create \
+			--rm \
+			--name=matrix-sliding-sync \
+			--log-driver=none \
+			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
+			--read-only \
+			--tmpfs /tmp \
+			--network={{ matrix_sliding_sync_container_network }} \
+			--env-file={{ matrix_sliding_sync_base_path }}/env \
+			--label-file={{ matrix_sliding_sync_base_path }}/labels \
+			{% for arg in matrix_sliding_sync_container_extra_arguments %}
+			{{ arg }} \
+			{% endfor %}
+			{{ matrix_sliding_sync_container_image }}
+
+{% for network in matrix_sliding_sync_container_additional_networks %}
+ExecStartPre={{ devture_systemd_docker_base_host_command_docker }} network connect {{ network }} matrix-sliding-sync
+{% endfor %}
+
+ExecStart={{ devture_systemd_docker_base_host_command_docker }} start --attach matrix-sliding-sync
+
+ExecStop=-{{ devture_systemd_docker_base_host_command_sh }} -c '{{ devture_systemd_docker_base_host_command_docker }} kill matrix-sliding-sync 2>/dev/null || true'
+ExecStop=-{{ devture_systemd_docker_base_host_command_sh }} -c '{{ devture_systemd_docker_base_host_command_docker }} rm matrix-sliding-sync 2>/dev/null || true'
+
+Restart=always
+RestartSec=30
+SyslogIdentifier=matrix-sliding-sync
+
+[Install]
+WantedBy=multi-user.target