Running `just add-inventory-host example.com 1.2.3.4` (or
`make add-inventory-host domain=example.com ip=1.2.3.4`) adds a new
host to the inventory, creating inventory/hosts and
inventory/host_vars/matrix.DOMAIN/vars.yml from the example files,
with strong secrets generated automatically.

Existing configuration is never overwritten. The command refuses to
run if the host is already in the inventory, so it can also be used
for adding more hosts later.

Based on the idea proposed in
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4682 by @Ser5

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.