Add support for heartbeat events in WebSocket client

[0xferit]

Description

This PR refactors the WebSocket client to expose heartbeat events to library users. Previously, heartbeat messages sent by the Bitfinex WebSocket server were silently consumed and filtered out, preventing users from monitoring connection health or implementing custom logic based on heartbeat patterns. Addressing #252 .

The implementation now:

• Emits heartbeat events for both public subscriptions and authenticated connections
• Provides subscription context for public channel heartbeats
• Maintains clean separation between subscription-based and authenticated heartbeats
• Preserves all existing functionality without breaking changes

Motivation and Context

Heartbeat events are essential for:

1. Connection Health Monitoring: Users can track if the WebSocket connection is alive and receiving data
2. Custom Reconnection Logic: Applications can implement sophisticated reconnection strategies based on heartbeat patterns
3. Analytics and Debugging: Developers can monitor connection quality and diagnose network issues
4. Transparency: Users gain full visibility into WebSocket communication

Currently, these events were being consumed internally but not exposed, limiting users' ability to implement robust connection management.

Related Issue

PR addresses the need for exposing heartbeat events that were previously only consumed internally.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Usage Examples

Public Channel Heartbeats

@bfx.wss.on("heartbeat")
def on_heartbeat(subscription):
    print(f"Heartbeat for {subscription['channel']} - {subscription['symbol']}")