Add Ci::Minutes::AdditionalPack Grape entity

In order to return additional pack objects to API requests, we
require a new entity to represent the new model.

ee/lib/ee/api/entities/ci/minutes/additional_pack.rb

+# frozen_string_literal: true
+
+module EE
+  module API
+    module Entities
+      module Ci
+        module Minutes
+          class AdditionalPack < Grape::Entity
+            expose :namespace_id
+            expose :expires_at
+            expose :number_of_minutes
+            expose :purchase_xid
+          end
+        end
+      end
+    end
+  end
+end

ee/spec/factories/ci/minutes/additional_packs.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :ci_minutes_additional_pack, class: 'Ci::Minutes::AdditionalPack' do
+    namespace
+    number_of_minutes { 10_000 }
+    expires_at { Date.current + 1.year }
+    purchase_xid { SecureRandom.hex(16) }
+  end
+end

ee/spec/lib/ee/api/entities/ci/minutes/additional_pack_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ::EE::API::Entities::Ci::Minutes::AdditionalPack do
+  it 'contains the correct attributes', :aggregate_failures do
+    pack = build(:ci_minutes_additional_pack)
+    entity = described_class.new(pack).as_json
+
+    expect(entity[:expires_at]).to eq pack.expires_at
+    expect(entity[:namespace_id]).to eq pack.namespace_id
+    expect(entity[:number_of_minutes]).to eq pack.number_of_minutes
+    expect(entity[:purchase_xid]).to eq pack.purchase_xid
+  end
+end