CCDM: Provide an API to get the Request in Endpoints

[manolo]

VaadinRequest.getCurrent is null in @Endpoints methods because they are handled by a different controller, it is causing user issues like is described in this comment https://vaadin.com/docs/v15/flow/typescript/configuring-security.html

There might be different solutions:

• make VaadinConnectController put the HttpRequest available in ThreadLocal using the vaadin CurrentInstance API

@Endpoint
public class MyEndpoint {

   @AnonymousAllowed
   public String echo(String parameter) {
       HttpRequest request = CurrentInstance.get(HttpRequest.class);
       if ( request.getUserPrincipal() != null) {
           return request.getUserPrincipal().getName();
       } 
      return anonymous;
   }
}

• add some magic API to the controller so as it can inject the request when needed by the user

@Endpoint
public class MyEndpoint {

   @AnonymousAllowed
   public String echo(String parameter, HttpRequest request) {
       if ( request.getUserPrincipal() != null) {
           return request.getUserPrincipal().getName();
       } 
      return anonymous;
   }
}

At the same time update the wrong snippet in the documentation wich is wrong https://vaadin.com/docs/v15/flow/typescript/configuring-security.html#accessing-user-information-in-an-endpoint-method

[Legioth]

Just like all other parts of Vaadin, we also want the endpoints to use the VaadinRequest and VaadinResponse abstractions so that your endpoint logic can be the same regardless of whether the runtime environment is Servlet, Portlet or in the future maybe something like Netty.

[manolo]

Decided in grooming to go with Leif's solution.
We don't need to do any change in the documentation, neither we add any additional API.