Network Bridge Refactoring (#1535)

* rename protocol ID to network capability

* guide: `ProtocolId` -> `NetworkCapability`

* guide: remove `RegisterEventProducer`

* capabilities and expand on underlying network assumptions

* guide: create network.md types file

* guide: network bridge is aware of network messages

* revert changes in code

* Update roadmap/implementers-guide/src/SUMMARY.md

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

* remove references to NetworkCapability

* Update roadmap/implementers-guide/src/types/network.md

Co-authored-by: Sergei Shulepov <sergei@parity.io>

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>
Co-authored-by: Sergei Shulepov <sergei@parity.io>

polkadot/roadmap/implementers-guide/src/node/backing/pov-distribution.md

@@ -4,15 +4,14 @@ This subsystem is responsible for distributing PoV blocks. For now, unified with
 
 ## Protocol
 
-`ProtocolId`: `b"povd"`, `PeerSet`: `Validation`
+`PeerSet`: `Validation`
 
 Input: [`PoVDistributionMessage`](../../types/overseer-protocol.md#pov-distribution-message)
 
 
 Output:
 
-- NetworkBridge::RegisterEventProducer(`ProtocolId`)
-- NetworkBridge::SendMessage(`[PeerId]`, `ProtocolId`, `Bytes`)
+- NetworkBridge::SendMessage(`[PeerId]`, message)
 - NetworkBridge::ReportPeer(PeerId, cost_or_benefit)
 
 
@@ -56,18 +55,7 @@ struct PeerState {
 }
 ```
 
-We also assume the following network messages, which are sent and received by the [Network Bridge](../utility/network-bridge.md)
-
-```rust
-enum NetworkMessage {
-	/// Notification that we are awaiting the given PoVs (by hash) against a
-	/// specific relay-parent hash.
-	Awaiting(Hash, Vec<Hash>),
-	/// Notification of an awaited PoV, in a given relay-parent context.
-	/// (relay_parent, pov_hash, pov)
-	SendPoV(Hash, Hash, PoV),
-}
-```
+We also use the [`PoVDistributionV1Message`](../../types/network.md#pov-distribution) as our `NetworkMessage`, which are sent and received by the [Network Bridge](../utility/network-bridge.md)
 
 Here is the logic of the state machine: