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/node/core/runtime-api/src/tests.rs

@@ -47,6 +47,7 @@ struct MockSubsystemClient {
 	validation_outputs_results: HashMap<ParaId, bool>,
 	session_index_for_child: SessionIndex,
 	candidate_pending_availability: HashMap<ParaId, CommittedCandidateReceipt>,
+	candidates_pending_availability: HashMap<ParaId, Vec<CommittedCandidateReceipt>>,
 	dmq_contents: HashMap<ParaId, Vec<InboundDownwardMessage>>,
 	hrmp_channels: HashMap<ParaId, BTreeMap<ParaId, Vec<InboundHrmpMessage>>>,
 	validation_code_by_hash: HashMap<ValidationCodeHash, ValidationCode>,
@@ -140,6 +141,14 @@ impl RuntimeApiSubsystemClient for MockSubsystemClient {
 		Ok(self.candidate_pending_availability.get(&para_id).cloned())
 	}
 
+	async fn candidates_pending_availability(
+		&self,
+		_: Hash,
+		para_id: ParaId,
+	) -> Result<Vec<CommittedCandidateReceipt<Hash>>, ApiError> {
+		Ok(self.candidates_pending_availability.get(&para_id).cloned().unwrap_or_default())
+	}
+
 	async fn candidate_events(&self, _: Hash) -> Result<Vec<CandidateEvent<Hash>>, ApiError> {
 		Ok(self.candidate_events.clone())
 	}