Delegate fields to associations

[Paul-Bob]

Add the possibility to delegate fields to associations. For example on post resource:

# app/avo/resources/post.rb
class Avo::Resources::Post < Avo::BaseResource
  def fields
    # ...
    field :user_name, for_attribute: "user.first_name"
    # ...
  end
end

Workaround

# app/models/post.rb
class Post < ApplicationRecord
  # just a writer for avo to render the filed on the form
  attr_writer :user_first_name
end

# app/avo/resources/post.rb
class Avo::Resources::Post < Avo::BaseResource
  def fields
    # ...
    field :user_first_name,
      format_using: -> { record.user.first_name },
      update_using: -> { record.user.update!(first_name: value) }
    # ...
  end
end