Add work item type field to custom field form

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/179912



Merged-by: Deepika Guliani <dguliani@gitlab.com>
Approved-by: Rajan Mistry <rmistry@gitlab.com>
Approved-by: Deepika Guliani <dguliani@gitlab.com>
Reviewed-by: Simon Knox <simon@gitlab.com>
Reviewed-by: Deepika Guliani <dguliani@gitlab.com>
Reviewed-by: Rajan Mistry <rmistry@gitlab.com>
Co-authored-by: Simon Knox <simon@gitlab.com>