Add support for passphrase to publicKey()

[FrankRuben]

Hi, I wanted to run fx, but I'm using a passphrase for my ssh key - and that was not yet supported by go-ssh-key. This PR changes the publicKey method to support. It is not production ready and holds some comments that might need reading, but it should be a starting point. And thanks for fx - it's really a great idea for a minimal and still flexible FaaS. Regards, Frank