Integrate litep2p into Polkadot SDK (#2944)

[litep2p](https://github.com/altonen/litep2p) is a libp2p-compatible P2P
networking library. It supports all of the features of `rust-libp2p`
that are currently being utilized by Polkadot SDK.

Compared to `rust-libp2p`, `litep2p` has a quite different architecture
which is why the new `litep2p` network backend is only able to use a
little of the existing code in `sc-network`. The design has been mainly
influenced by how we'd wish to structure our networking-related code in
Polkadot SDK: independent higher-levels protocols directly communicating
with the network over links that support bidirectional backpressure. A
good example would be `NotificationHandle`/`RequestResponseHandle`
abstractions which allow, e.g., `SyncingEngine` to directly communicate
with peers to announce/request blocks.

I've tried running `polkadot --network-backend litep2p` with a few
different peer configurations and there is a noticeable reduction in
networking CPU usage. For high load (`--out-peers 200`), networking CPU
usage goes down from ~110% to ~30% (80 pp) and for normal load
(`--out-peers 40`), the usage goes down from ~55% to ~18% (37 pp).

These should not be taken as final numbers because:

a) there are still some low-hanging optimization fruits, such as
enabling [receive window
auto-tuning](https://github.com/libp2p/rust-yamux/pull/176), integrating
`Peerset` more closely with `litep2p` or improving memory usage of the
WebSocket transport
b) fixing bugs/instabilities that incorrectly cause `litep2p` to do less
work will increase the networking CPU usage
c) verification in a more diverse set of tests/conditions is needed

Nevertheless, these numbers should give an early estimate for CPU usage
of the new networking backend.

This PR consists of three separate changes:
* introduce a generic `PeerId` (wrapper around `Multihash`) so that we
don't have use `NetworkService::PeerId` in every part of the code that
uses a `PeerId`
* introduce `NetworkBackend` trait, implement it for the libp2p network
stack and make Polkadot SDK generic over `NetworkBackend`
  * implement `NetworkBackend` for litep2p

The new library should be considered experimental which is why
`rust-libp2p` will remain as the default option for the time being. This
PR currently depends on the master branch of `litep2p` but I'll cut a
new release for the library once all review comments have been
addresses.

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>
Co-authored-by: Alexandru Vasile <alexandru.vasile@parity.io>

polkadot/node/network/collator-protocol/src/collator_side/tests/mod.rs

@@ -45,8 +45,9 @@ use polkadot_node_subsystem::{
 use polkadot_node_subsystem_test_helpers as test_helpers;
 use polkadot_node_subsystem_util::{reputation::add_reputation, TimeoutExt};
 use polkadot_primitives::{
-	AuthorityDiscoveryId, CollatorPair, ExecutorParams, GroupIndex, GroupRotationInfo, IndexedVec,
-	NodeFeatures, ScheduledCore, SessionIndex, SessionInfo, ValidatorId, ValidatorIndex,
+	AuthorityDiscoveryId, Block, CollatorPair, ExecutorParams, GroupIndex, GroupRotationInfo,
+	IndexedVec, NodeFeatures, ScheduledCore, SessionIndex, SessionInfo, ValidatorId,
+	ValidatorIndex,
 };
 use polkadot_primitives_test_helpers::TestCandidateBuilder;
 use test_helpers::mock::new_leaf;
@@ -249,10 +250,14 @@ fn test_harness<T: Future<Output = TestHarness>>(
 	let genesis_hash = Hash::repeat_byte(0xff);
 	let req_protocol_names = ReqProtocolNames::new(&genesis_hash, None);
 
-	let (collation_req_receiver, req_v1_cfg) =
-		IncomingRequest::get_config_receiver(&req_protocol_names);
-	let (collation_req_v2_receiver, req_v2_cfg) =
-		IncomingRequest::get_config_receiver(&req_protocol_names);
+	let (collation_req_receiver, req_v1_cfg) = IncomingRequest::get_config_receiver::<
+		Block,
+		sc_network::NetworkWorker<Block, Hash>,
+	>(&req_protocol_names);
+	let (collation_req_v2_receiver, req_v2_cfg) = IncomingRequest::get_config_receiver::<
+		Block,
+		sc_network::NetworkWorker<Block, Hash>,
+	>(&req_protocol_names);
 	let subsystem = async {
 		run_inner(
 			context,