Refactor RequestHandler to handle request component construction

This refactors RequestHandler so that it handles the construction of the request
component itself, rather than being handed a pre-built request component
instance constructed by the invoking servlet.

The motivation for this change is so that RequestHandler can be extended in
future CLs to compute authentication results, and can provide those results as
an available binding in the constructed request component.  An alternative
approach could have been to compute the authentication results within
RequestModule itself, but I think it's clearer to keep business logic like
that outside of Dagger providers.

This CL makes the following individual changes:

- Adds request component builders, which implement a RequestComponentBuilder
  interface so they can all be manipulated by RequestHandler

- Instead of obtaining request components via factory methods on the global
  components, one now can have global-scoped bindings just inject the request
  component builders (which requires adding a module to each global component
  declaring the subcomponent).  This follows the recommended approach here:
  http://google.github.io/dagger/subcomponents.html

- Instead of exposing request components on the global component interface,
  we now expose module-specific subclasses of RequestHandler that @Inject the
  appropriate request component builder's provider and pass it to the superclass
  (note that inheritance isn't strictly necessary here but saves boilerplate)

- RequestHandler now takes the Provider<RequestComponentBuilder> and builds
  the component itself using its own fresh RequestModule instance. This provides
  some nice encapsulation but is mainly needed for adding a RequestAuthModule
  in future work.

- RequestHandler also takes UserService now, which can be provided via Dagger
  by the subclass.  Longer-term that will go away in favor of instead providing
  AuthStrategy instances, some of which will use UserService internally.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138815648

java/google/registry/module/frontend/FrontendServlet.java

@@ -15,8 +15,6 @@
 package google.registry.module.frontend;
 
 import google.registry.monitoring.metrics.MetricReporter;
-import google.registry.request.RequestHandler;
-import google.registry.request.RequestModule;
 import google.registry.util.FormattingLogger;
 import java.io.IOException;
 import java.security.Security;
@@ -31,12 +29,10 @@ import org.bouncycastle.jce.provider.BouncyCastleProvider;
 public final class FrontendServlet extends HttpServlet {
 
   private static final FrontendComponent component = DaggerFrontendComponent.create();
+  private static final FrontendRequestHandler requestHandler = component.requestHandler();
   private static final MetricReporter metricReporter = component.metricReporter();
   private static final FormattingLogger logger = FormattingLogger.getLoggerForCallerClass();
 
-  private static final RequestHandler<FrontendRequestComponent> requestHandler =
-      RequestHandler.create(FrontendRequestComponent.class);
-
   @Override
   public void init() {
     Security.addProvider(new BouncyCastleProvider());
@@ -61,6 +57,6 @@ public final class FrontendServlet extends HttpServlet {
 
   @Override
   public void service(HttpServletRequest req, HttpServletResponse rsp) throws IOException {
-    requestHandler.handleRequest(req, rsp, component.startRequest(new RequestModule(req, rsp)));
+    requestHandler.handleRequest(req, rsp);
   }
 }