Add optional ReactiveX adapter for device events

[bruno-f-cruz]

Leaving this as a draft as it is a bit opinionated, but since I had this convo with @Poofjunior and @glopesdev separately, just putting it on a PR for future reference. This is how I would like to handle events from devices :P

Summary

tldr: reactive framework rocks — this is the optional Rx layer on top of the push-subscription API.

The core push-subscription API (Device.subscribe, Device.subscribe_all, the dedicated event thread) has already landed on refactor-unified-api directly, since it doesn't need ReactiveX. This PR is now scoped to just the optional Rx adapter on top of it.

What changed

• Optional Rx adapter (harp.device.rx) — observe(device, register) / observe_all(device) return hot, ref-counted (ops.share) Observables backed by a single underlying Device.subscribe/subscribe_all call. Gated behind the harp-device[rx] extra; importing the module without reactivex raises a clear ImportError. If I had it my way, we would only have this, but for now leaving it optional.
• Example — scripts/event_monitor.py shows the Rx adapter alongside the plain subscribe_all API on a live device (COM3). (A non-Rx-only version of this example also lives at docs/examples/subscribing_to_events/.)

Notes

• observe/observe_all emit on the device's shared event thread — use ops.observe_on(scheduler) to move heavy downstream work off it.