add --mandatory-headers-only cli option to complex relay (#1129)

bridges/relays/bin-substrate/src/cli/relay_headers_and_messages.rs

@@ -57,6 +57,9 @@ pub struct HeadersAndMessagesSharedParams {
 	lane: Vec<HexLaneId>,
 	#[structopt(long, possible_values = RelayerMode::VARIANTS, case_insensitive = true, default_value = "rational")]
 	relayer_mode: RelayerMode,
+	/// If passed, only mandatory headers (headers that are changing the GRANDPA authorities set) are relayed.
+	#[structopt(long)]
+	only_mandatory_headers: bool,
 	#[structopt(flatten)]
 	prometheus_params: PrometheusParams,
 }
@@ -278,6 +281,7 @@ impl RelayHeadersAndMessages {
 				right_transactions_mortality,
 				LeftToRightFinality::new(right_client.clone(), right_sign.clone()),
 				MAX_MISSING_LEFT_HEADERS_AT_RIGHT,
+				params.shared.only_mandatory_headers,
 			);
 			let right_to_left_on_demand_headers = OnDemandHeadersRelay::new(
 				right_client.clone(),
@@ -285,6 +289,7 @@ impl RelayHeadersAndMessages {
 				left_transactions_mortality,
 				RightToLeftFinality::new(left_client.clone(), left_sign.clone()),
 				MAX_MISSING_RIGHT_HEADERS_AT_LEFT,
+				params.shared.only_mandatory_headers,
 			);
 
 			// Need 2x capacity since we consider both directions for each lane