Nested Form Fields

How do you achieve nested form fields (like: [inputs_for/2](https://hexdocs.pm/phoenix_html/Phoenix.HTML.Form.html#inputs_for/2))?

Ex: When creating User, I also want to create contacts

Schema
```elixir
schema "users" do
field :username, :string

# Associations
has_many(:contacts, Contact)
end

def assoc_changeset(user, attrs \\ %{}) do
user
|> cast(attrs, [:username])
|> cast_assoc(:contacts, with: &Contact.changeset/2)
end
```

Kaffy
```elixir

defmodule MyApp.UserAdmin do

def create_changeset(schema, attrs) do
MyApp.User.assoc_changeset(schema, attrs)
end

def form_fields(_) do
[
username: %{label: "Username (Required)"},
contacts: %{
# IS THIS POSSIBLE?
}
]
end

end

```

Can this be achieved just by using form_fields