Implement process/socket-based PT support

We should implement the socket-based pluggable transport protocol. Spec (but we want to implement what is actually used and implemented by existing PTs, if the spec is not up to date).

This may involve introducing some new Rust trait to abstract away the existing machinery. Or perhaps the existing TcpProvider will be sufficient? Right now, anyway, we do not propose for this to become a stable API for Arti.

This is the first step towards #69 (closed) (as an MVP). In the future we will want to firm up and formalise the Rust API and provide an FFI API and so on.