There's some way to set the `source` field param for nested fields?

## Context

I'm currently trying to set my model user `User` subfield `user.actor.id` as field source of my `DjangoObjectType`.

This happens because there's other kind of users within the application like an `Organization` or even a `Bot` (all of them are actors).

## Example

Lets define a custom `Type`:

```py
class User(DjangoObjectType):
class Meta:
model = UserModel
exclude = ['actor']
filter_fields = ['actor__login']
interfaces = [graphene.relay.Node]
skip_registry = True

# The login field is originated from `actor` relationship, not from the user model itself.
# It doesn't work, actor__login is interpreted as user field [user.actor__login] not [user.actor.login].
login = graphene.Field(graphene.String, source='actor__login')
# ...
```

In the example above I define the login field source as `source='actor__login'` but I [saw that here](https://github.com/graphql-python/graphene/blob/dfece7f65d4bad51a87a8e9f03f9acef1e25d3f8/graphene/types/field.py#L16-L20) we're using `getattr` function to resolve the source so it's currently impossible to set the field without defining a custom resolver.

https://github.com/graphql-python/graphene/blob/dfece7f65d4bad51a87a8e9f03f9acef1e25d3f8/graphene/types/field.py#L16-L20

I would like to know if there's an alternative solution to select a subfield like the example or we're forced to create a custom resolver by using the `resolver=lambda _, __: ...` arg or the class method declaration.

Thanks for the hard work and the library!

Start with graphene/types/field.py at the referenced source-resolution code and trace how DjangoObjectType fields use the source value. Compare the `actor__login` example with the current custom-resolver alternative; done means the intended nested relationship behavior is supported or the limitation and expected resolver approach are documented.

索引モデルが issue の本文から書いたものです。