Add support for SSH interactive authentication

[exactly-one-kas]

For lifetime reasons, Cred now doesn't always carry a git_cred and therefore Cred::unwrap() may now panic if called on a Cred created using Cred::ssh_interactive(); this is the only backwards-incompatible change

Without leaking memory or memorizing created handlers in RemoteCallbacks it's not possible to have individual handler for each SSH interactive Cred, so I compromized and put the handler directly into RemoteCallbacks

Also, since libgit2 only ships with dummy typedefs for LIBSSH2_USERAUTH_KBDINT_PROMPT and LIBSSH2_USERAUTH_KBDINT_PROMPT, libssh2-sys is now a direct dependency for the ssh feature