Allow SSH/SFTP without a public key

[ldub]

If you pass in a null pointer for the public_key, libssh2 itself will generate a public key from the private key:

https://github.com/libssh2/libssh2/blob/482055695011fabb55f3cb7598356d47a0429068/src/userauth.c#L1520-L1530

It would be extremely handy to allow this in libssh2-hs.

Would you consider implementing this or accepting a PR if I did it?

[portnov]

Hello.
Yes, a PR would be welcome.