Expose ClaimQueue via a runtime api and use it in collation-generation (#3580)

The PR adds two things:
1. Runtime API exposing the whole claim queue
2. Consumes the API in `collation-generation` to fetch the next
scheduled `ParaEntry` for an occupied core.

Related to https://github.com/paritytech/polkadot-sdk/issues/1797

polkadot/node/core/runtime-api/src/lib.rs

@@ -177,6 +177,9 @@ where
 				self.requests_cache.cache_async_backing_params(relay_parent, params),
 			NodeFeatures(session_index, params) =>
 				self.requests_cache.cache_node_features(session_index, params),
+			ClaimQueue(relay_parent, sender) => {
+				self.requests_cache.cache_claim_queue(relay_parent, sender);
+			},
 		}
 	}
 
@@ -329,6 +332,8 @@ where
 					Some(Request::NodeFeatures(index, sender))
 				}
 			},
+			Request::ClaimQueue(sender) =>
+				query!(claim_queue(), sender).map(|sender| Request::ClaimQueue(sender)),
 		}
 	}
 
@@ -626,5 +631,11 @@ where
 			sender,
 			result = (index)
 		),
+		Request::ClaimQueue(sender) => query!(
+			ClaimQueue,
+			claim_queue(),
+			ver = Request::CLAIM_QUEUE_RUNTIME_REQUIREMENT,
+			sender
+		),
 	}
 }