Support user SSH keys for authentication

If I understand what I've seen so far correctly it appears that cloudflared only works with its own keypair. I would like to suggest supporting user keypairs. In particular, I'd like to see support for keypairs where the private key is stored on a hardware device (like a Yubikey). Instead of signing the internally generated key, you'd instead provide a short-lived certificate for the yubikey's public key, and then that certificate would be used to authenticate.

For an example of how this works, see [Pritunl Zero's implementation](https://docs.pritunl.com/docs/pritunl-zero#).