[Full-stack Signals] Add basic support for NumberSignals

[taefi]

Describe your motivation

There should be support for exposing a @BrowserCallable service containing public methods returning a NumberSignal subscription type as follows:

@BrowserCallable
@AnonymousAllowed
public class CounterService {
  private final NumberSignal counter = new NumberSignal(0);

  public NumberSignal counter() {
    return counter;
  }
}

in a way that the necessary TS codes are generated so that the user code can contain usages like this:

import { CounterService } from "Frontend/generated/endpoints";

const counter = CounterService.counter();

export default function CounterView() {
  return <Button onClick={() => counter.increment(1)}>Click count: {counter}</Button>
}

Describe the solution you'd like

N/A

Describe alternatives you've considered

This very first step can ignore as many complexities as possible such as:

• disconnecting/reconnecting the Websocket connection when moving away from the view of having connection problems
• having an event queue on the server to resolve the concurrency issues and etc.
• support for latency compensation

Additional context

The following modules are targeted for adding this feature:

• TS: react-signals
• Java: endpoint (under the com.vaadin.hilla.signals)

[vaadin-bot]

This ticket/PR has been released with Hilla 24.5.0.alpha6 and is also targeting the upcoming stable 24.5.0 version.