Adding RSA Support to swift-nio-ssh

Hi, author here. I needed to support RSA .pem keys to support SSH access to older Amazon EC2 instances, but swift-nio-ssh deliberately doesn't support them, so I had to roll my own and ran into a few gotchas worth noting and sharing. Long story short: There's a fork (complete with the latest security advisories) available of swift-no-ssh with said RSA support now and the blog post describes the changes. Happy to answer any questions!