Configure the scope on has many fields

Created by: zamith

Why:

• Many times we want to select from a list of resources that is scope. For instance, a user might be scoped by role.

This change addresses the need by:

• Allowing an option to be passed to the HasMany field, with a proc to be run

Usage:

ATTRIBUTE_TYPES = {
  admins: Field::HasMany.with_options(class_name: "User", scope: -> { User.admins }),
}

Fixes #331 (closed)