Support github id based public key encryption

[ysmood]

I want to achieve something like this:

sops --encrypt --whisper @rsc conf.yaml

The line above will encrypt the conf.yaml for github users rsc, every user's public key is always public available on github, such as https://github.com/rsc.keys, just append the .keys to a user's profile page url.

With this style, we don't have ask the recipient to generate new keys and send it to the maintainer, as long as they can push the repo they can decrypt the sops file, able to push means already has a public key uploaded to github. Same works for other platforms like gitlab.

I made a Age like tool for it: whisper.

My question is it easy to add new encryption tool to sops, are there guidelines or convention I need to follow to make a PR? I have read the CONTRIBUTING.md.

I'm aware of that #1134 is a year old, also sops age does't support key passphrase yet.

FYI: The comparison between age and whisper.