certbot-auto was removed from the certbot repo by EFF in 2021 (see issue
#204), which leaves certbot_install_method: source broken — the existing
install-from-source.yml still references {{ certbot_dir }}/certbot-auto,
a path that no longer exists after the git clone.
This adds a new install method `pip` that follows EFF's recommended
install path (https://certbot.eff.org/instructions?os=pip): create a
Python venv at {{ certbot_dir }}, pip install certbot, symlink the
binary onto PATH at /usr/local/bin/certbot.
Plugins (e.g. certbot-dns-rfc2136, certbot-dns-cloudflare) can be
opted into per-host via a new certbot_pip_extra_packages list, which
installs additional pip packages into the same venv after certbot.
The legacy `source` method is left in place for backwards compatibility
but is now documented as deprecated in the README and defaults file.
Tested on Debian 12 and Debian 13.
