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-base/templates/static-files/well-known/matrix-client.j2

@@ -26,6 +26,11 @@
 		"preferredDomain": {{ matrix_client_element_jitsi_preferredDomain|to_json }}
 	}
 	{% endif %}
+	{% if matrix_sliding_sync_enabled %},
+	"org.matrix.msc3575.proxy": {
+		"url": "{{ matrix_sliding_sync_base_url }}"
+	}
+	{% endif %}
 	{% if matrix_client_element_location_sharing_enabled %},
 	"m.tile_server": {
         "map_style_url": "https://{{ matrix_server_fqn_element }}/map_style.json"