feat: add more type hints

fastapi_cache/key_builder.py

@@ -1,18 +1,18 @@
 import hashlib
-from typing import Optional
+from typing import Optional, Callable
 
 from starlette.requests import Request
 from starlette.responses import Response
 
 
 def default_key_builder(
-    func,
+    func: Callable,
     namespace: Optional[str] = "",
     request: Optional[Request] = None,
     response: Optional[Response] = None,
     args: Optional[tuple] = None,
     kwargs: Optional[dict] = None,
-):
+) -> str:
     from fastapi_cache import FastAPICache
 
     prefix = f"{FastAPICache.get_prefix()}:{namespace}:"