Implement Flag Change Event System

[oasaph]

Description

Add support for notifications when flag values change, allowing applications to react to flag updates in real-time.

Goals

• Event-based notification system
• Support for multiple listeners
• Granular event filtering

Proposed Features

1. Flag change events (created, updated, deleted)
2. Value change events
3. Bulk update events
4. Error events

Example Interface

@flaggle.on_flag_change("feature_a")
def handle_feature_change(flag_name: str, old_value: Any, new_value: Any):
    print(f"Flag {flag_name} changed from {old_value} to {new_value}")

# Or class-based
class FlagListener:
    def on_change(self, flag_name: str, old_value: Any, new_value: Any):
        pass

flaggle.add_listener(FlagListener())

Acceptance Criteria

• Event system implementation
• Support for multiple listeners
• Filter by flag name/pattern
• Documentation and examples