Declarative site-to-site Wireguard configuration

[airone01]

I want to make a declarative site-to-site Wireguard configuration that automatically configures a network between my machines.
One consideration is that the IPs of cassiopeia and cetus, like all others, must not be made public, and can rotate at any moment (thanks to my ISP). So that's a problem.
An attempt was already done in feat/wireguard, but I'm starting over in dev.

I have taken most of my inspiration from @Xe's blog to make this, so thanks to them.

• https://xeiaso.net/blog/my-wireguard-setup-2021-02-06/
• https://xeiaso.net/blog/series/site-to-site-wireguard/