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/network/Cargo.toml

@@ -16,12 +16,16 @@ workspace = true
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]
 
+[build-dependencies]
+prost-build = "0.11"
+
 [dependencies]
 array-bytes = "6.1"
 async-channel = "1.8.0"
 async-trait = "0.1.79"
 asynchronous-codec = "0.6"
 bytes = "1"
+cid = "0.9.0"
 codec = { package = "parity-scale-codec", version = "3.6.1", features = ["derive"] }
 either = "1.5.3"
 fnv = "1.0.6"
@@ -42,17 +46,23 @@ smallvec = "1.11.0"
 thiserror = { workspace = true }
 tokio = { version = "1.22.0", features = ["macros", "sync"] }
 tokio-stream = "0.1.7"
-unsigned-varint = { version = "0.7.1", features = ["asynchronous_codec", "futures"] }
+unsigned-varint = { version = "0.7.2", features = ["asynchronous_codec", "futures"] }
 zeroize = "1.4.3"
 prometheus-endpoint = { package = "substrate-prometheus-endpoint", path = "../../utils/prometheus" }
+prost = "0.11"
 sc-client-api = { path = "../api" }
 sc-network-common = { path = "common" }
+sc-network-types = { path = "types" }
 sc-utils = { path = "../utils" }
 sp-arithmetic = { path = "../../primitives/arithmetic" }
 sp-blockchain = { path = "../../primitives/blockchain" }
 sp-core = { path = "../../primitives/core" }
 sp-runtime = { path = "../../primitives/runtime" }
 wasm-timer = "0.2"
+litep2p = { git = "https://github.com/paritytech/litep2p", branch = "master" }
+once_cell = "1.18.0"
+void = "1.0.2"
+schnellru = "0.2.1"
 
 [dev-dependencies]
 assert_matches = "1.3"
@@ -63,8 +73,11 @@ tempfile = "3.1.0"
 tokio = { version = "1.22.0", features = ["macros"] }
 tokio-util = { version = "0.7.4", features = ["compat"] }
 tokio-test = "0.4.2"
+sc-block-builder = { path = "../block-builder" }
 sc-network-light = { path = "light" }
 sc-network-sync = { path = "sync" }
+sp-crypto-hashing = { path = "../../primitives/crypto/hashing" }
+sp-consensus = { path = "../../primitives/consensus/common" }
 sp-test-primitives = { path = "../../primitives/test-primitives" }
 sp-tracing = { path = "../../primitives/tracing" }
 substrate-test-runtime = { path = "../../test-utils/runtime" }