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/uninstall.yml

@@ -0,0 +1,25 @@
+---
+
+- name: Check existence of matrix-sliding-sync service
+  ansible.builtin.stat:
+    path: "{{ devture_systemd_docker_base_systemd_path }}/matrix-sliding-sync.service"
+  register: matrix_sliding_sync_service_stat
+
+- when: matrix_sliding_sync_service_stat.stat.exists | bool
+  block:
+    - name: Ensure matrix-sliding-sync is stopped
+      ansible.builtin.service:
+        name: matrix-sliding-sync
+        state: stopped
+        enabled: false
+        daemon_reload: true
+
+    - name: Ensure matrix-sliding-sync.service doesn't exist
+      ansible.builtin.file:
+        path: "{{ devture_systemd_docker_base_systemd_path }}/matrix-sliding-sync.service"
+        state: absent
+
+    - name: Ensure matrix-sliding-sync paths don't exist
+      ansible.builtin.file:
+        path: "{{ matrix_sliding_sync_base_path }}"
+        state: absent