Implement Pluggable Storage Backends

[oasaph]

Description

Add support for loading flags from various sources beyond HTTP endpoints, making the library more versatile for different deployment scenarios.

Goals

• Support multiple storage backends
• Allow easy addition of new backends
• Maintain consistent API across backends

Proposed Storage Backends

1. File System (JSON, YAML)
2. Environment Variables
3. Redis
4. Database (SQLite, PostgreSQL)
5. etcd/Consul

Example Interface

class FlagStorage(ABC):
    @abstractmethod
    async def load_flags(self) -> dict:
        pass
    
    @abstractmethod
    async def save_flags(self, flags: dict) -> None:
        pass

flaggle = Flaggle(storage=FileSystemStorage("flags.json"))

Acceptance Criteria

• Abstract storage interface
• At least 2 storage implementations
• Documentation for each backend
• Migration guide for existing users