Integrate with systemd, provide Ubuntu PPA

`dnsproxy` is very useful. It would be great to have it fully integrated with `systemd` and available as an Ubuntu PPA.

Or in a more general way to make it easy to install and configure on multiple platforms, like `nextdns` CLI does.

Start by reviewing how the nextdns CLI handles installation and configuration, then define the scope for systemd integration and Ubuntu PPA packaging across platforms. Done means dnsproxy has an agreed, easy installation and configuration path with the requested systemd and Ubuntu support.

由索引模型根据 Issue 内容生成。