Implement PoV Distribution Subsystem (#1344)

* introduce candidatedescriptor type

* add PoVDistribution message type

* loosen bound on PoV Distribution to account for equivocations

* re-export some types from the messages module

* begin PoV Distribution subsystem

* remove redundant index from PoV distribution

* define state machine for pov distribution

* handle overseer signals

* set up control flow

* remove `ValidatorStatement` section

* implement PoV fetching

* implement distribution logic

* add missing `

* implement some network bridge event handlers

* stub for message processing, handle our view change

* control flow for handling messages

* handle `awaiting` message

* handle any incoming PoVs and redistribute

* actually provide a subsystem implementation

* remove set-builder notation

* begin testing PoV distribution

* test that we send awaiting messages only to peers with same view

* ensure we distribute awaited PoVs to peers on view changes

* test that peers can complete fetch and are rewarded

* test some reporting logic

* ensure peer is reported for flooding

* test punishing peers diverging from awaited protocol

* test that we eagerly complete peers' awaited PoVs based on what we receive

* test that we prune the awaited set after receiving

* expand pov-distribution in guide to match a change I made

* remove unneeded import

polkadot/Cargo.lock

@@ -4540,6 +4540,25 @@ dependencies = [
  "sp-wasm-interface",
 ]
 
+[[package]]
+name = "polkadot-pov-distribution"
+version = "0.1.0"
+dependencies = [
+ "assert_matches",
+ "futures 0.3.5",
+ "futures-timer 3.0.2",
+ "log 0.4.8",
+ "parity-scale-codec",
+ "parking_lot 0.10.2",
+ "polkadot-node-primitives",
+ "polkadot-node-subsystem",
+ "polkadot-primitives",
+ "polkadot-subsystem-test-helpers",
+ "sc-network",
+ "sp-runtime",
+ "streamunordered",
+]
+
 [[package]]
 name = "polkadot-primitives"
 version = "0.8.14"