Add signals

[rmcdaniel]

This PR adds the ability to await on a condition and signal a workflow.

For example:

class TestWorkflow extends Workflow
{
    private bool $canceled = false;

    #[SignalMethod]
    public function cancel()
    {
        $this->canceled = true;
    }

    public function execute()
    {
        yield WorkflowStub::await(fn () => $this->canceled);

        return;
    }
}

This workflow will reach the await() call and then pause until some external code signals the workflow like this:

$workflow->cancel();