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()`.

examples/in_memory/main.py

@@ -1,3 +1,5 @@
+from typing import Dict, Optional
+
 import pendulum
 import uvicorn
 from fastapi import FastAPI
@@ -84,9 +86,9 @@ app.get("/method")(cache(namespace="test")(instance.handler_method))
 # cache a Pydantic model instance; the return type annotation is required in this case
 class Item(BaseModel):
     name: str
-    description: str | None = None
+    description: Optional[str] = None
     price: float
-    tax: float | None = None
+    tax: Optional[float] = None
 
 
 @app.get("/pydantic_instance")
@@ -110,7 +112,7 @@ async def uncached_put():
 @cache(namespace="test", expire=5, injected_dependency_namespace="monty_python")
 def namespaced_injection(
     __fastapi_cache_request: int = 42, __fastapi_cache_response: int = 17
-) -> dict[str, int]:
+) -> Dict[str, int]:
     return {
         "__fastapi_cache_request": __fastapi_cache_request,
         "__fastapi_cache_response": __fastapi_cache_response,