Add observer/observable pattern to device

[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

I added a quick example that compares the two APIs

Summary

tldr: push over pull, reactive framework rocks.

Replaces the single overridable _on_event hook with a proper push-subscription API for device-emitted messages, delivered off the reader thread, plus an optional ReactiveX adapter.

What changed

• Device.subscribe(register, handler, *, message_types=Event) — register a callback that receives a typed, parsed ParsedHarpMessage. Filters by message type (default: unsolicited Events only; opt into Read/Write replies). Returns a Subscription handle with unsubscribe() (also a context manager).
• Device.subscribe_all(handler, ...) — catch-all across every address for logging/forwarding.
• Reader thread stays lean. _dispatch correlates request/reply replies on the fast path (O(1), non-blocking) and fans every message onto a dedicated event thread. Handlers run there, so a slow handler can't stall synchronous read()/write(), and handlers may safely call back into the device.
• close() clears subscriptions so a closed/reopened device retains no stale handlers.
• Optional Rx adapter (harp.device.rx) — observe() / observe_all() return hot, ref-counted (ops.share) Observables backed by a single underlying subscription. 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 prints all events from a device on COM3.

Notes

• Subscribers share one event thread → handlers are invoked sequentially, in order; keep them quick or offload with ops.observe_on(scheduler).