Improve type resolving

[Maximilien-R]

Since tartiflette allow us to deal only with dict. Resolving the __typename field for an UnionType is difficult and is delegate to the Resolver implementation which need to return the __typename in the resolved value.

Maybe we could add an optional resolve_type argument to the Resolver decorator which could take a callable in charge of determining the __typename to return for the resolved value? This resolve_type parameter could be required for an UnionType resolver which returns a value where the __typename couldn't be resolved by the default resolve_type function (a dict without a __typename key for instance).

@Resolver(
    "Query.catOrDog",
    resolve_type=lambda result: "Dog" if is_dog(result) else "Cat",
)
def query_cat_or_dog_resolver(*_args, **_kwargs):
    return {
        "name": "Dogo",
    }

[abusi]

I kind of like this idea @Maximilien-R.
@tsunammis what do you think of this idea?

I think it's great to enable our users to override the current default behavior.