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()`.

pyproject.toml

@@ -22,7 +22,7 @@ redis = { version = "^4.2.0rc1", optional = true }
 aiomcache = { version = "*", optional = true }
 pendulum = "*"
 aiobotocore = { version = "^1.4.1", optional = true }
-typing-extensions = { version = ">=4.1.0", markers = "python_version < \"3.10\"" }
+typing-extensions = { version = ">=4.1.0" }
 aiohttp = { version = ">=3.8.3", markers = "python_version >= \"3.11\"" }
 
 [tool.poetry.group.dev.dependencies]