Remove Parent Hash to Session mapping (#928)

* Adds a SigningContext type

* Bump spec versions

* Fixes requested changes

* Bump ParachainHost api_version and guard signing_context call

* Improve error message

* If there is no signing_context api use default value

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

polkadot/primitives/src/parachain.rs

@@ -608,6 +608,15 @@ pub enum ValidityAttestation {
 	Explicit(ValidatorSignature),
 }
 
+/// A type returned by runtime with current session index and a parent hash.
+#[derive(Clone, Eq, PartialEq, Default, Decode, Encode, RuntimeDebug)]
+pub struct SigningContext {
+	/// Current session index.
+	pub session_index: sp_staking::SessionIndex,
+	/// Hash of the parent.
+	pub parent_hash: Hash,
+}
+
 /// An attested candidate. This is submitted to the relay chain by a block author.
 #[derive(Clone, PartialEq, Decode, Encode, RuntimeDebug)]
 pub struct AttestedCandidate {
@@ -655,7 +664,7 @@ impl FeeSchedule {
 
 sp_api::decl_runtime_apis! {
 	/// The API for querying the state of parachains on-chain.
-	#[api_version(2)]
+	#[api_version(3)]
 	pub trait ParachainHost {
 		/// Get the current validators.
 		fn validators() -> Vec<ValidatorId>;
@@ -673,6 +682,8 @@ sp_api::decl_runtime_apis! {
 		/// Extract the abridged head that was set in the extrinsics.
 		fn get_heads(extrinsics: Vec<<Block as BlockT>::Extrinsic>)
 			-> Option<Vec<AbridgedCandidateReceipt>>;
+		/// Get a `SigningContext` with current `SessionIndex` and parent hash.
+		fn signing_context() -> SigningContext;
 	}
 }