Add sample: worker thread pool for Activities

Using https://github.com/piscinajs/piscina

Background

Activities are in the main thread. If you’re doing CPU-intensive work in Activities, I’d recommend doing them inside worker threads that you create.

https://github.com/temporalio/sdk-typescript/blob/main/docs/arch.svg

Make sure to hook up cancellation to the thread pool:

• https://github.com/piscinajs/piscina#cancelable-tasks
• https://typescript.temporal.io/api/classes/activity.Context#cancellationsignal

Heartbeats become more complicated when running in a different thread, the easiest way to achieve that is by periodically emitting empty heartbeats in the main thread, but you won't be able to report progress this way. If you need more advance heartbeat behavior you can use MessageChannel or Atomics.notify or bypass the worker heartbeat mechanism and emit heartbeats yourself with an AsyncCompletionClient.

Start with Piscina's cancelable-tasks documentation and the Temporal Activity Context cancellationSignal API linked in the issue. Build a TypeScript sample showing CPU-intensive Activities in a Piscina worker thread pool, with cancellation connected to the pool and a documented heartbeat approach. Done means the sample demonstrates the requested worker, cancellation, and heartbeat behavior.