proto: Reexport the Incoming* types from tor_proto::stream

This moves the pub re-exports of the incoming stream-related types from tor_proto::client::stream to tor_proto::stream.

This reorg is needed because currently, the only public export of these types is from tor_proto::client::stream, but none of them aren't actually client specific: relays will use them too, for implementing exit, DNS and directory streams. So it makes more sense to export them from the top-level stream module instead.

There are a few other types we need to move out of client (for example DataStream), but that can happen in a separate MR as it will require some code motion/refactoring.

This is in preparation for #1567