GraphQL: expose milestone `iid`

Problem

Currently Gitlab GraphQL API is not exposing Milestones' iid, this fields are required for a better migration.

Proposal

• Add the iid Milestone Type (https://gitlab.com/gitlab-org/gitlab/blob/master/app/graphql/types/milestone_type.rb)

    field :iid, GraphQL::ID_TYPE, null: false,
          description: 'Internal ID of the milestone.'

Documentation

Following documentation should be updated with the fields added in this issue:

• User docs: https://docs.gitlab.com/ee/user/group/import/