Ease parachain candidate code fetching (#2593)

* code stored in para + modify CandidateDescriptor.

* WIP: digest + some more impl

* validation_code_hash in payload + check in inclusion

* check in client + refator

* tests

* fix encoding indices

* remove old todos

* fix test

* fix test

* add test

* fetch validation code inside collation-generation from the relay-chain

* HashMismatch -> PoVHashMismatch + miscompilation

* refactor, store hash when needed

* storage rename: more specific but slightly too verbose

* do not hash on candidate validation, fetch hash instead

* better test

* fix test

* guide updates

* don't panic in runtime

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

polkadot/runtime/westend/src/lib.rs

@@ -983,6 +983,10 @@ sp_api::impl_runtime_apis! {
 		) -> BTreeMap<Id, Vec<InboundHrmpMessage<BlockNumber>>> {
 			BTreeMap::new()
 		}
+
+		fn validation_code_by_hash(_hash: Hash) -> Option<ValidationCode> {
+			None
+		}
 	}
 
 	impl beefy_primitives::BeefyApi<Block, BeefyId> for Runtime {