Add support for SSH keys stored on security keys

I'm trying to push to tangled using the SSH key bound to my machine. It seems that sk-ecdsa-sha2-nistp256@openssh.com keys are not supported. Other FIDO2 keys are likely not supported either, but I didn't verify that.

I verified that I am able to push with a normal ed25519 key.

Would be great, if you could support these keys!

Thanks, Jan :)