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/parachains/src/inclusion/mod.rs

@@ -1104,6 +1104,24 @@ impl<T: Config> Pallet<T> {
 		})
 	}
 
+	/// Returns all the `CommittedCandidateReceipt` pending availability for the para provided, if
+	/// any.
+	pub(crate) fn candidates_pending_availability(
+		para: ParaId,
+	) -> Vec<CommittedCandidateReceipt<T::Hash>> {
+		<PendingAvailability<T>>::get(&para)
+			.map(|candidates| {
+				candidates
+					.into_iter()
+					.map(|candidate| CommittedCandidateReceipt {
+						descriptor: candidate.descriptor.clone(),
+						commitments: candidate.commitments.clone(),
+					})
+					.collect()
+			})
+			.unwrap_or_default()
+	}
+
 	/// Returns the metadata around the first candidate pending availability for the
 	/// para provided, if any.
 	pub(crate) fn pending_availability(

polkadot/runtime/parachains/src/runtime_api_impl/v10.rs

@@ -301,6 +301,10 @@ pub fn validation_code<T: initializer::Config>(
 }
 
 /// Implementation for the `candidate_pending_availability` function of the runtime API.
+#[deprecated(
+	note = "`candidate_pending_availability` will be removed. Use `candidates_pending_availability` to query 
+	all candidates pending availability"
+)]
 pub fn candidate_pending_availability<T: initializer::Config>(
 	para_id: ParaId,
 ) -> Option<CommittedCandidateReceipt<T::Hash>> {

polkadot/runtime/parachains/src/runtime_api_impl/vstaging.rs

@@ -16,8 +16,8 @@
 
 //! Put implementations of functions from staging APIs here.
 
-use crate::scheduler;
-use primitives::{CoreIndex, Id as ParaId};
+use crate::{inclusion, initializer, scheduler};
+use primitives::{CommittedCandidateReceipt, CoreIndex, Id as ParaId};
 use sp_runtime::traits::One;
 use sp_std::{
 	collections::{btree_map::BTreeMap, vec_deque::VecDeque},
@@ -41,3 +41,11 @@ pub fn claim_queue<T: scheduler::Config>() -> BTreeMap<CoreIndex, VecDeque<ParaI
 		})
 		.collect()
 }
+
+/// Returns all the candidates that are pending availability for a given `ParaId`.
+/// Deprecates `candidate_pending_availability` in favor of supporting elastic scaling.
+pub fn candidates_pending_availability<T: initializer::Config>(
+	para_id: ParaId,
+) -> Vec<CommittedCandidateReceipt<T::Hash>> {
+	<inclusion::Pallet<T>>::candidates_pending_availability(para_id)
+}