Support ssh-ed25519 key format

[kubrickfr]

Since https://github.com/Netflix/bless/pull/74 got merged and clients 7.8 have problems connecting to servers <7.8:

https://bugzilla.redhat.com/show_bug.cgi?id=1623929 https://bugs.archlinux.org/task/59838 https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1790963

And that no solution seems to be on the horizon to resolve this backward compatibility issue, now might be the right time to support ed25519 key signing which, fortunately, is not very difficult on the client side.

This PR does not inclue suggesting that ed25519 should the default in README.md as this would be providing advice on the cryptographic methods.

[kubrickfr]

The CI failure seems to be a CI problem, not a problem with the code...

[tuxinaut]

Is there any reason why this can't be merged? We ran into the above mentioned problem.