grandpa: always try to import available block justifcation (#3928)

* grandpa: always try to import justifications in blocks

* grandpa: export useful types

* grandpa: add test for justification import on regular blocks

* grandpa: expand comment in test

substrate/core/finality-grandpa/src/justification.rs

@@ -39,7 +39,7 @@ use crate::communication;
 /// This is meant to be stored in the db and passed around the network to other
 /// nodes, and are used by syncing nodes to prove authority set handoffs.
 #[derive(Encode, Decode)]
-pub(crate) struct GrandpaJustification<Block: BlockT> {
+pub struct GrandpaJustification<Block: BlockT> {
 	round: u64,
 	pub(crate) commit: Commit<Block>,
 	votes_ancestries: Vec<Block::Header>,