Decode cache data to the correct endpoint type

[mjpieters]

Use the return annotation to decode cached data to the correct type.
This follows the same logic FastAPI uses to JSON request bodies.

For the PickleCoder, this is a no-op as pickle already stores type
information in the serialised data.

Fixes #120

[long2ice]

Thanks!

[mjpieters]

One concern I do think I'd like to address still is that of performance. Creating a Pydantic field for a return type is fast, but not so fast that caching the result won't help. Given that there are a finite number of endpoints that you want to apply caching to, but an infinite number of cache hits, I think a type -> field cache would help here. I'll do some testing.