Add Async/Await Support for Flag Operations

[oasaph]

Description

Implement async/await support for flag fetching and updates to improve performance in async applications and prevent blocking operations.

Goals

• Async flag fetching
• Non-blocking update operations
• Compatible with popular async frameworks
• Maintain sync API for backward compatibility

Proposed Features

1. Async Flaggle client
2. Async storage operations
3. Async event listeners
4. Integration with async web frameworks

Example Interface

from python_flaggle import AsyncFlaggle

async def main():
    flaggle = AsyncFlaggle(
        url="https://api.example.com/flags",
        interval=60
    )
    
    # Async flag checks
    if await flaggle.flags["feature_a"].is_enabled():
        await enable_feature()
    
    # Async update operations
    await flaggle.update_flags()

# Framework integration
app = FastAPI()

@app.get("/feature")
async def check_feature():
    return {"enabled": await flaggle.flags["feature"].is_enabled()}

Acceptance Criteria

• Async Flaggle implementation
• Async storage operations
• Async event system
• Integration examples with FastAPI/aiohttp
• Performance benchmarks
• Migration guide for existing users