add provider to user serializer

app/serializers/user_serializer.rb

 class UserSerializer < ActiveModel::Serializer
   cache key: 'user', expires_in: 4.hours
-  attributes :id, :email, :name, :description, :role_ids, :institution_ids, :avatar, :likes_count, :learning_objects_count, :collections_count, :created_at, :updated_at
+  attributes :id, :email, :provider, :name, :description, :role_ids, :institution_ids, :avatar, :likes_count, :follows_count, :learning_objects_count, :collections_count, :created_at, :updated_at
 end