Add support for heartbeat events in WebSocket client

[0xferit]

Expose WebSocket heartbeat events

This PR exposes WebSocket heartbeat events to allow users to monitor connection health for both public and authenticated channels.

Changes

• Added heartbeat event to event emitter: Registered as a valid event type in BfxEventEmitter
• Exposed heartbeat events: Modified WebSocket client and bucket to emit heartbeat events instead of silently discarding them
• Consistent handler signature: Heartbeat handlers receive subscription: Optional[Subscription] parameter
  • For public channels: receives subscription details
  • For authenticated connection (channel 0): receives None
• Added comprehensive example: New examples/websocket/heartbeat.py demonstrates usage for both public and authenticated flows
• Updated documentation: Added heartbeat events section to README with proper type hints

Handler Signature

@bfx.wss.on("heartbeat")
def on_heartbeat(subscription: Optional[Subscription]) -> None:
    if subscription:
        # Public channel heartbeat
        print(f"Heartbeat for {subscription['channel']}")
    else:
        # Authenticated connection heartbeat
        print("Heartbeat on channel 0")