Deduplicate pallet call structs used for indirect runtime calls (#1744)

* Small changes

* Define generic bridge pallets call structs

* polkadot-core SignedExtension simplifications

- we don't seem to need to pass the Call as a generic param
- we can use codec(skip) instead of implementing Encode and Decode

* Split BridgeHubRococo and BridgeHubWococo calls

* code review fixes

bridges/relays/lib-substrate-relay/src/on_demand/headers.rs

@@ -316,7 +316,7 @@ async fn background_task<P: SubstrateFinalitySyncPipeline>(
 						stall_timeout,
 						only_mandatory_headers,
 					},
-					metrics_params.clone().unwrap_or_else(|| MetricsParams::disabled()),
+					metrics_params.clone().unwrap_or_else(MetricsParams::disabled),
 					futures::future::pending(),
 				)
 				.fuse(),