Downward Message Processing implementation (#1859)

* DMP: data structures and plumbing

* DMP: Implement DMP logic in the router module

DMP: Integrate DMP parts into the inclusion module

* DMP: Introduce the max size limit for the size of a downward message

* DMP: Runtime API for accessing inbound messages

* OCD

Small clean ups

* DMP: fix the naming of the error

* DMP: add caution about a non-existent recipient

polkadot/node/collation-generation/src/lib.rs

@@ -277,6 +277,7 @@ async fn handle_new_activations<Context: SubsystemContext>(
 					new_validation_code: collation.new_validation_code,
 					head_data: collation.head_data,
 					erasure_root,
+					processed_downward_messages: collation.processed_downward_messages,
 				};
 
 				let ccr = CandidateReceipt {
@@ -387,6 +388,7 @@ mod tests {
 				proof_of_validity: PoV {
 					block_data: BlockData(Vec::new()),
 				},
+				processed_downward_messages: Default::default(),
 			}
 		}