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/bin-substrate/src/cli/init_bridge.rs

@@ -176,7 +176,7 @@ impl BridgeInitializer for RococoToBridgeHubWococoCliBridge {
 		init_data: <Self::Engine as Engine<Self::Source>>::InitializationData,
 	) -> <Self::Target as Chain>::Call {
 		relay_bridge_hub_wococo_client::runtime::Call::BridgeRococoGrandpa(
-			relay_bridge_hub_wococo_client::runtime::BridgeGrandpaRococoCall::initialize(init_data),
+			relay_bridge_hub_wococo_client::runtime::BridgeRococoGrandpaCall::initialize(init_data),
 		)
 	}
 }