Add binding for `libssh2_userauth_keyboard_interactive_ex`

[kb]

There does not currently appear to be a binding for libssh2_userauth_keyboard_interactive_ex. This would be useful / required for establishing connections where userauth_password is unsupported.

[Stephen von Takach]

annoyingly the method in libssh2 would request input from STDIN which is not desirable (especially since we're using that for core -> driver comms)

So need to think of another way of supporting that...

[Stephen von Takach]

Actually I'm wrong, it does support a password callback.

[Stephen von Takach]

fix by 0ea5478

Spec written but doesn't work with the test container as I had to manually enable interactive auth... Will work something out.