Attach updated endpoint signature to inner

Not all endpoints accept a __signature__ attribute, nor should the
cache decorator modify the decorated endpoint. Attach the signature
to the returned inner function instead.

While here, refactor the signature updating code, and extract it to
a separate function.

examples/in_memory/main.py

@@ -67,6 +67,19 @@ async def cache_response_obj():
     return JSONResponse({"a": 1})
 
 
+class SomeClass:
+    def __init__(self, value):
+        self.value = value
+
+    async def handler_method(self):
+        return self.value
+
+
+# register an instance method as a handler
+instance = SomeClass(17)
+app.get("/method")(cache(namespace="test")(instance.handler_method))
+
+
 @app.on_event("startup")
 async def startup():
     FastAPICache.init(InMemoryBackend())