Some refactors (#1929)

Had to modify some tests for raw-payload
(#1927) + got some local
clippy lints. This frontloads taking care of those unrelated
improvements (and some more I found), so the next PR will have a smaller
diff.

Author: svix-jplatte

server/svix-server/tests/it/e2e_auth.rs

@@ -118,7 +118,7 @@ async fn test_invalid_auth_error_detail() {
                 .to_owned()
         }
 
-        _ => return {},
+        _ => return,
     };
 
     client.set_auth_header("some-nonsense-key".to_string());