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/node/core/runtime-api/src/lib.rs

@@ -556,6 +556,13 @@ mod tests {
 			) -> BTreeMap<ParaId, Vec<InboundHrmpMessage>> {
 				self.hrmp_channels.get(&recipient).map(|q| q.clone()).unwrap_or_default()
 			}
+
+			fn validation_code_by_hash(
+				&self,
+				_hash: Hash,
+			) -> Option<ValidationCode> {
+				unreachable!("not used in tests");
+			}
 		}
 
 		impl BabeApi<Block> for MockRuntimeApi {