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/tasks/validate_config.yml

@@ -0,0 +1,11 @@
+---
+- name: Fail if required matrix-sliding-sync settings not defined
+  ansible.builtin.fail:
+    msg: >
+      You need to define a required configuration setting (`{{ item.name }}`).
+  when: "vars[item] == ''"
+  with_items:
+    - matrix_sliding_sync_hostname
+    - matrix_sliding_sync_path_prefix
+    - matrix_sliding_sync_database_hostname
+    - matrix_sliding_sync_environment_variable_syncv3_secret