Re federate to instance when their domain name changes

The FHMY instance lost their domain (fmhy.ml) recently and therefore had to get a new domain name. They did not lose their data. However, this broke the federation between other instances.

• You can still access lemmy.fmhy.ml communities and posts on other instances such as lemmy.world even though lemmy.fmhy.ml is completely offline
• Even if FMHY used the data they had from lemmy.fmhy.ml in their new domain name lemmy.fmhy.net, it will just duplicate/fork the communities and posts from on other instances.

I think a good solution to solving this is to have each instance generate a public/private key pair, and when two instances federate, they will share public keys. If a instances goes offline and reappears again under a new domain name, it will know it is the same domain name due to the public key pair and re federate by changing the domain name on every community and post made with that instance.

Similar Github issues https://github.com/LemmyNet/lemmy/issues/3690