Decode cache data to the correct endpoint type

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.

examples/in_memory/main.py

@@ -7,6 +7,7 @@ from starlette.responses import JSONResponse, Response
 from fastapi_cache import FastAPICache
 from fastapi_cache.backends.inmemory import InMemoryBackend
 from fastapi_cache.decorator import cache
+from pydantic import BaseModel
 
 app = FastAPI()
 
@@ -80,6 +81,20 @@ instance = SomeClass(17)
 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
+    price: float
+    tax: float | None = None
+
+
+@app.get("/pydantic_instance")
+@cache(namespace="test", expire=5)
+async def pydantic_instance() -> Item:
+    return Item(name="Something", description="An instance of a Pydantic model", price=10.5)
+
+
 @app.on_event("startup")
 async def startup():
     FastAPICache.init(InMemoryBackend())