protocols/relay: Implement circuit relay v2 protocol (#2059)

This commit adds an implementation for the circuit relay v2 protocol to be used
as a relay server, i.e. it supports incoming HOP requests and outgoing STOP
requests and used as a relay clients, i.e. outgoing HOP requests and incoming
STOP requests.

The existing circuit relay v1 protocol implementation is moved to
protocols/relay/src/v1.

Co-authored-by: ronzigelman <ronzigelman@gmail.com>
Co-authored-by: Marco Munizaga <git@marcopolo.io>
Co-authored-by: Thomas Eizinger <thomas@eizinger.io>
Co-authored-by: Elena Frank <57632201+elenaf9@users.noreply.github.com>

protocols/relay/src/v2.rs

@@ -0,0 +1,41 @@
+// Copyright 2021 Protocol Labs.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+//! Implementation of the [libp2p circuit relay v2
+//! specification](https://github.com/libp2p/specs/issues/314).
+
+mod message_proto {
+    include!(concat!(env!("OUT_DIR"), "/message_v2.pb.rs"));
+}
+
+pub mod client;
+mod copy_future;
+mod protocol;
+pub mod relay;
+
+/// The ID of an outgoing / incoming, relay / destination request.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub struct RequestId(u64);
+
+impl RequestId {
+    fn new() -> RequestId {
+        RequestId(rand::random())
+    }
+}