Request based PoV distribution (#2640)

* Indentation fix.

* Prepare request-response for PoV fetching.

* Drop old PoV distribution.

* WIP: Fetch PoV directly from backing.

* Backing compiles.

* Runtime access and connection management for PoV distribution.

* Get rid of seemingly dead code.

* Implement PoV fetching.

Backing does not yet use it.

* Don't send `ConnectToValidators` for empty list.

* Even better - no need to check over and over again.

* PoV fetching implemented.

+ Typechecks
+ Should work

Missing:

- Guide
- Tests
- Do fallback fetching in case fetching from seconding validator fails.

* Check PoV hash upon reception.

* Implement retry of PoV fetching in backing.

* Avoid pointless validation spawning.

* Add jaeger span to pov requesting.

* Add back tracing.

* Review remarks.

* Whitespace.

* Whitespace again.

* Cleanup + fix tests.

* Log to log target in overseer.

* Fix more tests.

* Don't fail if group cannot be found.

* Simple test for PoV fetcher.

* Handle missing group membership better.

* Add test for retry functionality.

* Fix flaky test.

* Spaces again.

* Guide updates.

* Spaces.

polkadot/node/service/src/lib.rs

@@ -450,7 +450,6 @@ where
 	use polkadot_node_collation_generation::CollationGenerationSubsystem;
 	use polkadot_collator_protocol::{CollatorProtocolSubsystem, ProtocolSide};
 	use polkadot_network_bridge::NetworkBridge as NetworkBridgeSubsystem;
-	use polkadot_pov_distribution::PoVDistribution as PoVDistributionSubsystem;
 	use polkadot_node_core_provisioner::ProvisioningSubsystem as ProvisionerSubsystem;
 	use polkadot_node_core_runtime_api::RuntimeApiSubsystem;
 	use polkadot_statement_distribution::StatementDistribution as StatementDistributionSubsystem;
@@ -518,9 +517,6 @@ where
 			authority_discovery,
 			request_multiplexer,
 		),
-		pov_distribution: PoVDistributionSubsystem::new(
-			Metrics::register(registry)?,
-		),
 		provisioner: ProvisionerSubsystem::new(
 			spawner.clone(),
 			(),