Enable sending payments to fn invoke_transaction()

It needs to be possible to send value with the transaction,
because `t.transferred_value` in this function could be set
to > 0.

examples/multisig_plain/lib.rs

@@ -486,7 +486,7 @@ mod multisig_plain {
         ///
         /// Its return value indicates whether the called transaction was successful.
         /// This can be called by anyone.
-        #[ink(message)]
+        #[ink(message, payable)]
         pub fn invoke_transaction(
             &mut self,
             trans_id: TransactionId,