Add Support for Custom HTTP Client Implementation

[oasaph]

Description

Currently, the library is tightly coupled to requests.get for fetching flags. We should allow users to provide their own HTTP client implementation for more flexibility.

Goals

• Allow injection of custom HTTP client
• Support different HTTP libraries (aiohttp, httpx, etc.)
• Maintain backward compatibility

Proposed Implementation

1. Create an abstract base class for flag fetching
2. Implement the default requests-based fetcher
3. Allow custom fetcher injection in Flaggle constructor
4. Add documentation and examples

Example Interface

class FlagFetcher(ABC):
    @abstractmethod
    async def fetch_flags(self, url: str) -> dict:
        pass

flaggle = Flaggle(url="https://api.example.com/flags", fetcher=CustomFetcher())

Acceptance Criteria

• Abstract base class for flag fetching
• Default implementation using requests
• Documentation for custom implementations
• Unit tests for custom fetcher support