You can use Apigee to manage[1] the interactions between Dialogflow and your backend services. Apigiee's ParseDialogflowRequest and SetDialogflowResponse policies enable easier integration with Dialogflow[2]. By using these policies, you can easily plug-in your backend services to handle fulfillment requests from Dialogflow. Using Apigee you can use Graphql[3].
[1]https://cloud.google.com/apigee/docs/api-platform/publish/create-api-products#add
[3]https://cloud.google.com/apigee/docs/api-platform/develop/graphql