refactor+feat: allow subsystems to send only declared messages, generate graphviz (#5314)

Closes #3774
Closes #3826

polkadot/node/network/availability-distribution/src/pov_requester/mod.rs

@@ -27,7 +27,7 @@ use polkadot_node_primitives::PoV;
 use polkadot_node_subsystem::{
 	jaeger,
 	messages::{IfDisconnected, NetworkBridgeMessage},
-	SubsystemContext,
+	overseer,
 };
 use polkadot_node_subsystem_util::runtime::RuntimeInfo;
 use polkadot_primitives::v2::{AuthorityDiscoveryId, CandidateHash, Hash, ValidatorIndex};
@@ -39,6 +39,7 @@ use crate::{
 };
 
 /// Start background worker for taking care of fetching the requested `PoV` from the network.
+#[overseer::contextbounds(AvailabilityDistribution, prefix = self::overseer)]
 pub async fn fetch_pov<Context>(
 	ctx: &mut Context,
 	runtime: &mut RuntimeInfo,
@@ -48,10 +49,7 @@ pub async fn fetch_pov<Context>(
 	pov_hash: Hash,
 	tx: oneshot::Sender<PoV>,
 	metrics: Metrics,
-) -> Result<()>
-where
-	Context: SubsystemContext,
-{
+) -> Result<()> {
 	let info = &runtime.get_session_info(ctx.sender(), parent).await?.session_info;
 	let authority_id = info
 		.discovery_keys