Allow user to pass SocketProtector callback function for android

[zhangsan946]

Allow user to implement the Rust interface which calls the Java VPNService protect method, and to be used to protect the socket fd right after the socket is created on Android.

[zonyitoo]

Should SocketProtector defined as a trait?

[zhangsan946]

@zonyitoo there is no need, when user wanna pass a callback, all he needs to do is to create a SocketProtector with SocketProtector::new(move |fd| callback(fd)) and pass to ConnectOpts. If to be trait, it is expected user needs implement a dedicated struct just for this callback while the former is a much simpler implementation.

[zonyitoo]

No. Users don't need to really implement this trait if they only has a function, just add a helper function:

fn make_socket_protector<F>(f: F) -> FnSocketProtector<F> ...

which FnSocketProtector<F> is exactly the same as your SocketProtector implementation.

This design could be found in tower, hyper, ... which could allow users who doesn't want to simply call a function, they could make a "Functor" with states.