feat: Add injectAt extension method on Router<T>

lib/src/router/router.dart

@@ -232,6 +232,10 @@ extension RouteEx<T extends Object> on Router<T> {
     attach(path, subRouter);
     return subRouter;
   }
+
+  /// Injects [injectable] at [path]
+  void injectAt(final String path, final InjectableIn<Router<T>> injectable) =>
+      group(path).inject(injectable);
 }
 
 /// Just a typedef for better auto-complete

test/router/router_inject_test.dart

@@ -43,6 +43,26 @@ void main() {
     });
   });
 
+  group('Router.injectAt', () {
+    test('Given a HandlerObject, '
+        'when using injectAt, '
+        'then it is injected at the specified path', () async {
+      final router = Router<Handler>();
+      router.injectAt('/api', const _EchoHandlerObject(mountAt: '/echo'));
+
+      final request = Request(
+        Method.post,
+        Uri.parse('http://localhost/api/echo'),
+        body: Body.fromString('injected at path'),
+      );
+      final ctx = request.toContext(Object());
+      final result = await router.asHandler(ctx) as ResponseContext;
+
+      expect(result.response.statusCode, 200);
+      expect(await result.response.readAsString(), 'injected at path');
+    });
+  });
+
   group('Router.inject with MiddlewareObject', () {
     test(
       'Given a MiddlewareObject, '