Runtime API: introduce candidates_pending_availability (#4027)

Fixes https://github.com/paritytech/polkadot-sdk/issues/3576

Required by elastic scaling collators.
Deprecates old API: `candidate_pending_availability`.

TODO:
- [x] PRDoc

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/runtime/westend/src/lib.rs

@@ -1853,6 +1853,7 @@ sp_api::impl_runtime_apis! {
 		}
 
 		fn candidate_pending_availability(para_id: ParaId) -> Option<CommittedCandidateReceipt<Hash>> {
+			#[allow(deprecated)]
 			parachains_runtime_api_impl::candidate_pending_availability::<Runtime>(para_id)
 		}
 
@@ -1966,6 +1967,10 @@ sp_api::impl_runtime_apis! {
 		fn claim_queue() -> BTreeMap<CoreIndex, VecDeque<ParaId>> {
 			vstaging_parachains_runtime_api_impl::claim_queue::<Runtime>()
 		}
+
+		fn candidates_pending_availability(para_id: ParaId) -> Vec<CommittedCandidateReceipt<Hash>> {
+			vstaging_parachains_runtime_api_impl::candidates_pending_availability::<Runtime>(para_id)
+		}
 	}
 
 	impl beefy_primitives::BeefyApi<Block, BeefyId> for Runtime {