Implement a JSON tower layer

This will help requesting JSON APIs

crates/http/src/ext.rs

@@ -0,0 +1,30 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use crate::layers::json::Json;
+
+pub trait ServiceExt {
+    fn json<T>(self) -> Json<Self, T>
+    where
+        Self: Sized;
+}
+
+impl<S> ServiceExt for S {
+    fn json<T>(self) -> Json<Self, T>
+    where
+        Self: Sized,
+    {
+        Json::new(self)
+    }
+}