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/key_builder.py

@@ -1,5 +1,5 @@
 import hashlib
-from typing import Any, Callable, Optional
+from typing import Any, Callable, Dict, Optional, Tuple
 
 from starlette.requests import Request
 from starlette.responses import Response
@@ -10,8 +10,8 @@ def default_key_builder(
     namespace: str = "",
     request: Optional[Request] = None,
     response: Optional[Response] = None,
-    args: Optional[tuple[Any, ...]] = None,
-    kwargs: Optional[dict[str, Any]] = None,
+    args: Optional[Tuple[Any, ...]] = None,
+    kwargs: Optional[Dict[str, Any]] = None,
 ) -> str:
     cache_key = hashlib.md5(  # nosec:B303
         f"{func.__module__}:{func.__name__}:{args}:{kwargs}".encode()