Expose Label created_at and updated_at on GraphQL

Problem

Currently Gitlab GraphQL API is not exposing Labels' created_at and updated_at fields. These fields are required for a better migration.

Proposal

• Add the created_at and updated_at to the Label Type (https://gitlab.com/gitlab-org/gitlab/blob/master/app/graphql/types/label_type.rb)

    field :created_at, Types::TimeType, null: false,
          description: 'Timestamp of when the label was created.'
    field :updated_at, Types::TimeType, null: false,
          description: 'Timestamp of when the label was last updated.'