[EE] API: Allow to get and set "masked" attribute for variables
What does this MR do?
EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28381
Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/61754
EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28381
Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/61754