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>

substrate/client/consensus/common/Cargo.toml

@@ -19,7 +19,6 @@ targets = ["x86_64-unknown-linux-gnu"]
 async-trait = "0.1.79"
 futures = { version = "0.3.30", features = ["thread-pool"] }
 futures-timer = "3.0.1"
-libp2p-identity = { version = "0.1.3", features = ["ed25519", "peerid"] }
 log = { workspace = true, default-features = true }
 mockall = "0.11.3"
 parking_lot = "0.12.1"
@@ -27,6 +26,7 @@ serde = { features = ["derive"], workspace = true, default-features = true }
 thiserror = { workspace = true }
 prometheus-endpoint = { package = "substrate-prometheus-endpoint", path = "../../../utils/prometheus" }
 sc-client-api = { path = "../../api" }
+sc-network-types = { path = "../../network/types" }
 sc-utils = { path = "../../utils" }
 sp-api = { path = "../../../primitives/api" }
 sp-blockchain = { path = "../../../primitives/blockchain" }

substrate/client/consensus/common/src/import_queue.rs

@@ -64,7 +64,7 @@ pub type BoxJustificationImport<B> =
 	Box<dyn JustificationImport<B, Error = ConsensusError> + Send + Sync>;
 
 /// Maps to the RuntimeOrigin used by the network.
-pub type RuntimeOrigin = libp2p_identity::PeerId;
+pub type RuntimeOrigin = sc_network_types::PeerId;
 
 /// Block data used by the queue.
 #[derive(Debug, PartialEq, Eq, Clone)]

substrate/client/consensus/common/src/import_queue/basic_queue.rs

@@ -632,7 +632,7 @@ mod tests {
 			let hash = Hash::random();
 			finality_sender
 				.unbounded_send(worker_messages::ImportJustification(
-					libp2p_identity::PeerId::random(),
+					sc_network_types::PeerId::random(),
 					hash,
 					1,
 					(*b"TEST", Vec::new()),