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/subsystem-util/src/lib.rs

@@ -30,7 +30,7 @@ use polkadot_node_subsystem::{
 	messages::{RuntimeApiMessage, RuntimeApiRequest, RuntimeApiSender},
 	overseer, SubsystemSender,
 };
-use polkadot_primitives::{slashing, ExecutorParams};
+use polkadot_primitives::{slashing, CoreIndex, ExecutorParams};
 
 pub use overseer::{
 	gen::{OrchestraError as OverseerError, Timeout},
@@ -53,7 +53,10 @@ pub use rand;
 use sp_application_crypto::AppCrypto;
 use sp_core::ByteArray;
 use sp_keystore::{Error as KeystoreError, KeystorePtr};
-use std::time::Duration;
+use std::{
+	collections::{BTreeMap, VecDeque},
+	time::Duration,
+};
 use thiserror::Error;
 use vstaging::get_disabled_validators_with_fallback;
 
@@ -304,6 +307,7 @@ specialize_requests! {
 	fn request_submit_report_dispute_lost(dp: slashing::DisputeProof, okop: slashing::OpaqueKeyOwnershipProof) -> Option<()>; SubmitReportDisputeLost;
 	fn request_disabled_validators() -> Vec<ValidatorIndex>; DisabledValidators;
 	fn request_async_backing_params() -> AsyncBackingParams; AsyncBackingParams;
+	fn request_claim_queue() -> BTreeMap<CoreIndex, VecDeque<ParaId>>; ClaimQueue;
 }
 
 /// Requests executor parameters from the runtime effective at given relay-parent. First obtains