[BOT-272] ClientBuilder extend convenience function

reqwest-middleware/src/client.rs

@@ -31,6 +31,14 @@ impl ClientBuilder {
         }
     }
 
+    /// Convenience method to extend a ClientBuilder instance by appending
+    /// the middleware and initialiser stacks from a ClientwithMiddleware
+    pub fn extend(mut self, client_with_middleware: ClientWithMiddleware) -> Self {
+        self.middleware_stack.append(&mut client_with_middleware.middleware_stack.into_vec());
+        self.initialiser_stack.append(&mut client_with_middleware.initialiser_stack.into_vec());
+        self
+    }
+
     /// Convenience method to attach middleware.
     ///
     /// If you need to keep a reference to the middleware after attaching, use [`with_arc`].