Add support for Ed25519 keys (non SK)

[cipriancraciun]

It would be nice to also add support for Ed25519 (plain ones, not SK variants) keys.

The main intended use-case is with more limited SSH servers (usually in the embedded space) such as Dropbear (which only in 2022 has added support for -SK), tinysshd, or many other smaller implementations.

[cipriancraciun]

Looking at older tickets, perhaps this one is covered by #28, #18, or #15?

I don't think so, because I'm speaking of generating Ed25519 keys on the Android device, with the generate subcommand, not only loading local keys from ~/.ssh/... folder to keep compatibility with locally administered keys.

[3b0b]

This kind of functionality (albeit with RSA keys) is why I have held out using this project's predecessor to the bitter end, and don't have as much use for AKR as I otherwise would. The main utility for me is to make SSH connections (some of which are to servers or services that don't support -sk keys in the first place) and webauthn responses from a device I am remoted into (or which lacks USB ports completely), which makes using my physical security token...awkward at best, impossible in some cases.