title: Let Apigee do authentication (oauth) for client
body: <p>I have a service protected with OAuth. For it to use, you first require a token.
  I have an app, that has only access to an Apigee proxy. I would like Apigee to do the authentication for the client (app), and setup protection with an API key for the client in Apigee. How do I do that?</p>