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>

cumulus/client/relay-chain-rpc-interface/src/rpc_client.rs

@@ -655,6 +655,20 @@ impl RelayChainRpcClient {
 			.await
 	}
 
+	/// Get the receipt of all candidates pending availability.
+	pub async fn parachain_host_candidates_pending_availability(
+		&self,
+		at: RelayHash,
+		para_id: ParaId,
+	) -> Result<Vec<CommittedCandidateReceipt>, RelayChainError> {
+		self.call_remote_runtime_function(
+			"ParachainHost_candidates_pending_availability",
+			at,
+			Some(para_id),
+		)
+		.await
+	}
+
 	pub async fn validation_code_hash(
 		&self,
 		at: RelayHash,