Use DefaultSerde in more places

[igalshilman]

This PR adds more places to use the DefaultSerde and in addtion adds an optional type_hint argument in various ctx methods, that allows to use a more specific serializer.
For now we only support Pydantic and plain python objects via json.dumps/loads

class State(BaseModel):
   foo ...
   bar ...

res = await ctx.get("state", type_hint=State)

print(res.foo)