Merge branch '520781-only-fetch-custom-field-select-options-when-opening-dropdown' into 'master'

Only fetch custom field select options when opening dropdown

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182338



Merged-by: Coung Ngo <cngo@gitlab.com>
Approved-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Approved-by: Coung Ngo <cngo@gitlab.com>
Reviewed-by: Simon Knox <simon@gitlab.com>
Reviewed-by: Coung Ngo <cngo@gitlab.com>
Co-authored-by: Fernanda Toledo <ftoledo@gitlab.com>