Define keybuilder protocol

This lets others create key builders that are type checked fully.

fastapi_cache/types.py

@@ -0,0 +1,21 @@
+from typing import Any, Awaitable, Callable, Optional, Protocol, Union
+
+from starlette.requests import Request
+from starlette.responses import Response
+
+
+_Func = Callable[..., Any]
+
+
+class KeyBuilder(Protocol):
+    def __call__(
+        self,
+        _function: _Func,
+        _namespace: str = ...,
+        *,
+        request: Optional[Request] = ...,
+        response: Optional[Response] = ...,
+        args: Optional[tuple[Any, ...]] = ...,
+        kwargs: Optional[dict[str, Any]] = ...,
+    ) -> Union[Awaitable[str], str]:
+        ...