[C#] Integrate CancellationToken with ToSignal's AwaitableSignal Creation

[PereViader]

In the Godot engine, signals provide a mechanism to handle asynchronous scenarios. However, the method doesn't natively support any kind of cancellation. The idiomatic way of doing cancellation in c# is to use CancellationToken. The lack of compatibility of ToSignal with CancellationToken limits the developer's control over cancellation and resource management.

With this, we would align Godot’s async capabilities with common C# asynchronous programming practices, making it familiar and intuitive for C# developers.

Proposed usage

public class MyNode : Node
{
    public async Task PerformAsyncOperation(CancellationToken cancellationToken)
    {
        try
        {
            await ToSignal(this, "SomeSignal", cancellationToken);
            // Logic to execute after signal emission
        }
        catch (OperationCanceledException)
        {
            // Logic to handle cancellation, maybe cleanup or user notification
        }
    }
}

[PereViader]

A workaround for this is to implement the cancellation support manually for each of the Signals as extension methods.

A Roslyn source generator could be written to handle this.

public static Task AwaitSomeSignal(this MyNode node, CancellationToken ct = default)
{
  var tcs = new TaskCompletionSource();

  void Receive()
  {
     tcs.TrySetResult();
     node.SomeSignal -= Receive;
  }
  node.SomeSignal += Receive;
  if (ct.CanBeCanceled)
  {
    ct.Register(() => taskCompletionSource.TrySetCanceled(ct));
  }

  return tcs.Task;
}

The advantadge of doing it in this way is that given a signal that has some parameters, this could expose the parameters in a type safe way

int potato = await node.AwaitSomeSignal1();
(int potato, bool someCondition) = await node.AwaitSomeSignal2();