GRANDPA module: store accepted justifications (#2298) (#2301)

Store accepted justifications in events.

bridges/primitives/header-chain/src/lib.rs

@@ -21,7 +21,7 @@
 
 use bp_runtime::{
 	BasicOperatingMode, Chain, HashOf, HasherOf, HeaderOf, RawStorageProof, StorageProofChecker,
-	StorageProofError,
+	StorageProofError, UnderlyingChainProvider,
 };
 use codec::{Codec, Decode, Encode, EncodeLike, MaxEncodedLen};
 use core::{clone::Clone, cmp::Eq, default::Default, fmt::Debug};
@@ -234,3 +234,17 @@ pub trait ChainWithGrandpa: Chain {
 	/// refund amount and doing calls which exceed the limit, may be costly to submitter.
 	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32;
 }
+
+/// A trait that provides the type of the underlying `ChainWithGrandpa`.
+pub trait UnderlyingChainWithGrandpaProvider: UnderlyingChainProvider {
+	/// Underlying `ChainWithGrandpa` type.
+	type ChainWithGrandpa: ChainWithGrandpa;
+}
+
+impl<T> UnderlyingChainWithGrandpaProvider for T
+where
+	T: UnderlyingChainProvider,
+	T::Chain: ChainWithGrandpa,
+{
+	type ChainWithGrandpa = T::Chain;
+}