Mixnet integration (#1346)

See #1345, <https://github.com/paritytech/substrate/pull/14207>.

This adds all the necessary mixnet components, and puts them together in
the "kitchen-sink" node/runtime. The components added are:

- A pallet (`frame/mixnet`). This is responsible for determining the
current mixnet session and phase, and the mixnodes to use in each
session. It provides a function that validators can call to register a
mixnode for the next session. The logic of this pallet is very similar
to that of the `im-online` pallet.
- A service (`client/mixnet`). This implements the core mixnet logic,
building on the `mixnet` crate. The service communicates with other
nodes using notifications sent over the "mixnet" protocol.
- An RPC interface. This currently only supports sending transactions
over the mixnet.

---------

Co-authored-by: David Emett <dave@sp4m.net>
Co-authored-by: Javier Viola <javier@parity.io>

Cargo.toml

@@ -217,6 +217,7 @@ members = [
 	"substrate/client/keystore",
 	"substrate/client/merkle-mountain-range",
 	"substrate/client/merkle-mountain-range/rpc",
+	"substrate/client/mixnet",
 	"substrate/client/network-gossip",
 	"substrate/client/network",
 	"substrate/client/network/bitswap",
@@ -298,6 +299,7 @@ members = [
 	"substrate/frame/membership",
 	"substrate/frame/merkle-mountain-range",
 	"substrate/frame/message-queue",
+	"substrate/frame/mixnet",
 	"substrate/frame/multisig",
 	"substrate/frame/nft-fractionalization",
 	"substrate/frame/nfts",
@@ -394,6 +396,7 @@ members = [
 	"substrate/primitives/maybe-compressed-blob",
 	"substrate/primitives/merkle-mountain-range",
 	"substrate/primitives/metadata-ir",
+	"substrate/primitives/mixnet",
 	"substrate/primitives/npos-elections",
 	"substrate/primitives/npos-elections/fuzzer",
 	"substrate/primitives/offchain",