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/primitives/runtime/src/lib.rs

@@ -47,6 +47,7 @@ pub use storage_types::BoundedStorageValue;
 #[cfg(feature = "std")]
 pub use storage_proof::craft_valid_storage_proof;
 
+pub mod calls;
 pub mod messages;
 
 mod chain;