greedy relayer don't need message dispatch to be prepaid if dispatch is supposed to be paid at the target chain (#1016)

bridges/primitives/runtime/src/messages.rs

@@ -20,7 +20,7 @@ use codec::{Decode, Encode};
 use frame_support::{weights::Weight, RuntimeDebug};
 
 /// Where message dispatch fee is paid?
-#[derive(Encode, Decode, RuntimeDebug, Clone, PartialEq, Eq)]
+#[derive(Encode, Decode, RuntimeDebug, Clone, Copy, PartialEq, Eq)]
 pub enum DispatchFeePayment {
 	/// The dispacth fee is paid at the source chain.
 	AtSourceChain,