setting ed25519 ssh key as `user_public_key_path` will fail to ssh into nodes - causes a problem when node requires elliptic key

[slickwarren]

see https://github.com/rancherlabs/corral/blob/main/cmd/create.go#L358 for what I've found to be the culprit; tldr we expect the user to have an rsa key set, however some OS require an elliptic key (and others require an rsa key, like windows). We should have this function be dynamic enough to handle both rsa and elliptic keys