Feature Request: add ability to create "Masked and hidden" variables with gitlab_project_variable

Project variables can only be set to "Masked and hidden" if created that way, per docs: https://docs.gitlab.com/ee/ci/variables/index.html#hide-a-cicd-variable . The current resource gitlab_project_variable does not have a way to create these.

It would be really useful to be able to create these using terraform.

An additional feature request, which may be necessary to make this work anyhow, is to allow the value to remain unset from terraform, so the variable's value can be managed only in the UI, and doesn't need to be stored in terraform (or whatever secret store the terraform user is using).