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/bin/runtime-common/src/messages.rs

@@ -692,6 +692,13 @@ pub mod target {
 	}
 }
 
+/// The `BridgeMessagesCall` used by a chain.
+pub type BridgeMessagesCallOf<C> = bp_messages::BridgeMessagesCall<
+	bp_runtime::AccountIdOf<C>,
+	target::FromBridgedChainMessagesProof<bp_runtime::HashOf<C>>,
+	source::FromBridgedChainMessagesDeliveryProof<bp_runtime::HashOf<C>>,
+>;
+
 #[cfg(test)]
 mod tests {
 	use super::*;