Decouples light-sync state from chain spec (#9491)

* Decouples light-sync state from chain spec

This decouples the light-sync state from chain spec. First, the
light-sync state currently only works with BABE+Grandpa, so not
all *Substrate* based chains can use this feature. The next problem was
also that this pulled the `sc-consensus-babe` and `sc-finality-grandpa`
crate into `sc-chain-spec`.

If a chain now wants to support the light-sync state, it needs to add
the `LightSyncStateExtension` to the chain spec as an extension. This is
documented in the crate level docs of `sc-sync-state-rpc`. If this
extension is not available, `SyncStateRpc` fails at initialization.

* Fix compilation for browser

* Fmt

substrate/bin/node-template/node/src/service.rs

@@ -203,7 +203,7 @@ pub fn new_full(mut config: Configuration) -> Result<TaskManager, ServiceError>
 			let deps =
 				crate::rpc::FullDeps { client: client.clone(), pool: pool.clone(), deny_unsafe };
 
-			crate::rpc::create_full(deps)
+			Ok(crate::rpc::create_full(deps))
 		})
 	};
 
@@ -436,7 +436,7 @@ pub fn new_light(mut config: Configuration) -> Result<TaskManager, ServiceError>
 		transaction_pool,
 		task_manager: &mut task_manager,
 		on_demand: Some(on_demand),
-		rpc_extensions_builder: Box::new(|_, _| ()),
+		rpc_extensions_builder: Box::new(|_, _| Ok(())),
 		config,
 		client,
 		keystore: keystore_container.sync_keystore(),