Adding a concern for image size variants for ActiveStorage, similar on how paperclip gem used to do

example.rb

### Example of Use

class User < ApplicationRecord
  include ImageVariants
  has_one_attached :avatar
  image_variants_for :avatar, sizes: { custom_size: }, default_image: '/path/to/default/image'
end

image_variants_concern.rb

require 'active_support/concern'

module ImageVariants
  extend ActiveSupport::Concern

  class_methods do
    attr_reader :_attachment_name, :_image_sizes, :_default_image

    def image_variants_for(name, sizes: _default_sizes, default_image: _image)
      @_attachment_name = name
      @_image_sizes = sizes
      @_default_image = default_image
    end

    def _default_sizes
      {
        avatar: { combine_options: { resize: '80x80^', extent: '80x80', gravity: 'Center' } },
        thumb: { combine_options: { resize: '256x256^', crop: '128x128+0-10', gravity: 'Center' } },
        card: { combine_options: { resize: '640x360^', crop: '640x360+0+0', gravity: 'Center' } },
        cover: { combine_options: { resize: '1280x720^', crop: '1280x720+0+0', gravity: 'Center' } }
      }
    end

    def _image
      '/default.png'
    end
  end

  included do
    after_initialize do
      define_singleton_method("#{self.class._attachment_name}_image") do |size = nil|
        _attachment.attached? ? variant_image(size) : self.class._default_image
      end
    end
  end

  private

  def image_variants(size)
    self.class._image_sizes[size.to_sym]
  end

  def _attachment
    public_send(self.class._attachment_name)
  end

  def variant_image(size)
    size = image_variants(size)
    if size && _attachment.blob.variable?
      variant = _attachment.variant(size)
      variant.processed.service_url
    else
      _attachment.service_url
    end
  end
end