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

cumulus/client/relay-chain-minimal-node/src/blockchain_rpc_client.rs

@@ -14,7 +14,10 @@
 // You should have received a copy of the GNU General Public License
 // along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
 
-use std::pin::Pin;
+use std::{
+	collections::{BTreeMap, VecDeque},
+	pin::Pin,
+};
 
 use cumulus_relay_chain_interface::{RelayChainError, RelayChainResult};
 use cumulus_relay_chain_rpc_interface::RelayChainRpcClient;
@@ -25,6 +28,7 @@ use polkadot_primitives::{
 	async_backing::{AsyncBackingParams, BackingState},
 	slashing,
 	vstaging::{ApprovalVotingParams, NodeFeatures},
+	CoreIndex,
 };
 use sc_authority_discovery::{AuthorityDiscovery, Error as AuthorityDiscoveryError};
 use sc_client_api::AuxStore;
@@ -442,6 +446,13 @@ impl RuntimeApiSubsystemClient for BlockChainRpcClient {
 	async fn node_features(&self, at: Hash) -> Result<NodeFeatures, ApiError> {
 		Ok(self.rpc_client.parachain_host_node_features(at).await?)
 	}
+
+	async fn claim_queue(
+		&self,
+		at: Hash,
+	) -> Result<BTreeMap<CoreIndex, VecDeque<cumulus_primitives_core::ParaId>>, ApiError> {
+		Ok(self.rpc_client.parachain_host_claim_queue(at).await?)
+	}
 }
 
 #[async_trait::async_trait]