Typing cleanup

- Compatibility with older Python versions
  - use `Optional` and `Union` instead of `... | None` and `a | b`
  - use `typing_extensions.Protocol` instead of `typing.Protocol`
  - use `typing.Dict`, `typing.List`, etc. instead of the concrete types.

- Fix backend `.get()` annotations; not all were marked as `Optional[str]`
- Don't return anything from `Backend.set()` methods.
- The `Coder.decode_as_type()` type parameter must be a type to be
  compatible with `ModelField(..., type_=...)`.
- Clean up `Optional[]` use, remove where it is not needed.
- Clean up variable use in decorator, keeping the raw cached value
  separate from the return value from the wrapped endpoint.
- Annotate the wrapper as returning either the original type _or_ a
  Response (returning a 304 Not Modified response).
- Clean up small edge-case where `response` could be `None`.
- Correct type annotation on `JsonCoder.decode()` to match `Coder.decode()`.

fastapi_cache/types.py

@@ -1,7 +1,8 @@
-from typing import Any, Awaitable, Callable, Optional, Protocol, Union
+from typing import Any, Awaitable, Callable, Dict, Optional, Tuple, Union
 
 from starlette.requests import Request
 from starlette.responses import Response
+from typing_extensions import Protocol
 
 
 _Func = Callable[..., Any]
@@ -15,7 +16,7 @@ class KeyBuilder(Protocol):
         *,
         request: Optional[Request] = ...,
         response: Optional[Response] = ...,
-        args: tuple[Any, ...] = ...,
-        kwargs: dict[str, Any] = ...,
+        args: Tuple[Any, ...] = ...,
+        kwargs: Dict[str, Any] = ...,
     ) -> Union[Awaitable[str], str]:
         ...