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>

substrate/bin/node/cli/src/cli.rs

@@ -27,6 +27,10 @@ pub struct Cli {
 	#[clap(flatten)]
 	pub run: sc_cli::RunCmd,
 
+	#[allow(missing_docs)]
+	#[clap(flatten)]
+	pub mixnet_params: sc_cli::MixnetParams,
+
 	/// Disable automatic hardware benchmarks.
 	///
 	/// By default these benchmarks are automatically ran at startup and measure