Typing bug in FeatureManager.is_enabled overloads: single argument call not supported by type hints

• Package Version: 2.1.0
• Operating System: Ubuntu
• Python Version: 3.12

Describe the bug
The FeatureManager.is_enabled method has incorrect type hints in the published SDK.

The implementation accepts a single argument:

await feature_manager.is_enabled("my-flag")

This works at runtime because the method signature is:

async def is_enabled(self, feature_flag_id: str, *args, **kwargs) -> bool

However, the declared overloads do not include a signature that accepts only one argument. The only available overload is:

@overload
async def is_enabled(self, feature_flag_id: str, user_id: str, **kwargs) -> bool:
    ...

Type checkers such as Mypy report this usage as invalid and treat the return type as Any.

Expected Behavior
The type hints should describe all valid usages. Since the runtime implementation allows calling the method with only feature_flag_id, the overloads should include a matching variant.

Proposed Fix
Add a second overload:

@overload
async def is_enabled(self, feature_flag_id: str) -> bool:
    ...

This aligns the type hints with the implementation and returns the correct bool type to static analysis tools.

找出 FeatureManager.is_enabled 的多載宣告，並將其與此 issue 中的執行階段簽章和單一引數範例進行比較。加入缺少的單一引數型別標註變體，然後驗證靜態分析接受該呼叫並推斷出 bool。