Set up a unified registry servlet for Jetty (#2338)

This PR creates a unified RegistryServlet that will serve all
non-console traffic. It also creates a jetty subproject that allows one
to run Nomulus on top of a standard Jetty 12 runtime.

`./gradlew :jetty:stage` will create a jetty base folder at
`jetty/build/jetty-base` where one is able spin up a local Nomulus server
by running the following command inside the folder:

```bash
java -jar ${JETTY_HOME}/start.jar
```

`JETTY_HOME` is a folder where the [Jetty runtime](https://repo1.maven.org/maven2/org/eclipse/jetty/jetty-home/12.0.6/jetty-home-12.0.6.zip) is located.

This PR also adds a Gradle task to create a Nomulus image based on the
official Jetty image:

```bash
./gradlew :jetty:buildNomulusImage
```

core/src/test/java/google/registry/request/RequestHandlerTest.java

@@ -199,7 +199,7 @@ public final class RequestHandlerTest {
   void beforeEach() throws Exception {
     // Initialize here, not inline, so that we pick up the mocked UserService.
     handler =
-        RequestHandler.createForTest(
+        RequestHandler.create(
             Component.class,
             () ->
                 new Builder() {