Add a setting to enable display of pipeline variables
### Problem
Following the implementation of https://gitlab.com/gitlab-org/gitlab/-/issues/323097
We've received feedback from some of our users that would like to disable this feature
### Proposal
Adding a setting in the UI that enable this feature, once a user enable the feature a warning pops up mentioning that some users may be adding secrets in a manual variable which now will be visible in the UI
This setting will be available for project owners only
#### **UPDATED 2025: This issue is for the BE and FE work, which might or might not be in the same MR.**
issue