This commit adds copyright attirbutions in SPDX format to files for matrix-sliding-sync, following the REUSE's specification.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

Most of these files were defining a service, usually toward the end.
These lines have been moved upward.

Some components (mautrix-signal, mautrix-gmessages, etc.) were defining
a service conditionally (only if metrics are exposed, etc). This was
causing issues like these in the Traefik logs:

> level=error msg="service \"matrix-mautrix-twitter\" error: port is missing" providerName=docker container=matrix-mautrix-twitter-..

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