have a way to get underlying file descriptors

Currently the TCP/UDP types returned by a Runtime are opaque: there is no way to access the underlying file descriptor.
Access to that file descriptor is a requirement to implement transparent proxy (part of #72), and I suspect it would make onionmasq#23 (closed) easier (right now I think they'll have to implement their own TcpProvider, not wrapping arti default, to be able to get that).

It'll be kind of an issue with regard to crossplarformness: Windows has no FD, it has file handles which are mostly the same, but I guess different in some ways. Also some runtimes might not have backing fds for whatever reason, so returning a fd should probably be optional