SFTP: public key AND password auth?

Hi @hierynomus! It looks like there are some SFTP servers that require publickey and password based authentication. Glancing at the `auth*` methods in `SSHClient`, it's not immediately obvious to me if this is supported at all.

The corresponding manual/command-line way to do this is:

`$> sftp -vvv -oPort=123 -oIdentityFile=/path/to/key -oUser=username`

The above cmd says that key authentication succesful, trying password next (or something of the sort). Is there something obvious I'm missing - or is this not supported in the library as of now?