add registry entity and specs

app/serializers/project_registry_entity.rb

+class ProjectRegistryEntity < Grape::Entity
+  include ActionView::Helpers::NumberHelper
+
+  expose :project_id
+  expose :last_repository_synced_at
+  expose :last_repository_successful_sync_at
+  expose :last_wiki_synced_at
+  expose :last_wiki_successful_sync_at
+  expose :repository_retry_count
+  expose :wiki_retry_count
+  expose :last_repository_sync_failure
+  expose :last_wiki_sync_failure
+end

app/serializers/project_registry_serializer.rb

+class ProjectRegistrySerializer < BaseSerializer
+  entity ProjectRegistryEntity
+end

spec/serializers/project_registry_entity_spec.rb

+require 'spec_helper'
+
+describe ProjectRegistryEntity, :postgresql do
+  let(:registry) { create(:geo_project_registry, :synced) }
+
+  let(:entity) do
+    described_class.new(registry, request: double)
+  end
+
+  subject { entity.as_json }
+
+  it { is_expected.to have_key(:project_id) }
+  it { is_expected.to have_key(:last_repository_synced_at) }
+  it { is_expected.to have_key(:last_repository_successful_sync_at) }
+  it { is_expected.to have_key(:last_wiki_synced_at) }
+  it { is_expected.to have_key(:last_wiki_successful_sync_at) }
+  it { is_expected.to have_key(:repository_retry_count) }
+  it { is_expected.to have_key(:wiki_retry_count) }
+  it { is_expected.to have_key(:last_repository_sync_failure) }
+  it { is_expected.to have_key(:last_wiki_sync_failure) }
+end