[UI/UX] Add option in CI/CD settings for variable expansion

Description

In the .gitlab-ci.yml file, a user can define a variable and use it in another variable definition within the same .gitlab-ci.yml file. #1809 (closed)

Users also need to be able to allow nested variables through the applications' UI.

Proposal

⭐ SEE PROTOTYPES ON FIGMA ⭐

• Under project Settings > CI/CD > Variables, we will add an option for users to enable/disable nested variables in the ci file.
• frontend - A checkbox button is added to the section.
  • The option is not selected by default.
• frontend - Users need to click the save changes button to apply their option.
  • A feedback notification is displayed using the component:toast - https://design.gitlab.com/components/toast/
  • https://design.gitlab.com/product-foundations/saving-and-feedback
• Technical Writing - The UI text of the Variables section will also be updated.

Permissions and Security

Permissions should be consistent with the existing permissions structure regarding which roles can access project settings https://docs.gitlab.com/ee/user/permissions.html#project-members-permissions

• Members with no access (0) should not be able access the project settings
• Guest (10) members should not be able to access the project settings
• Reporter (20) members should not be able to access the project settings
• Developer (30) members should be able to access the project settings
• Maintainer (40) members should be able to see and interact with the option in the project settings
• Owner (50) members should be able to see and interact with the option in the project settings

Documentation

We should update https://docs.gitlab.com/ee/ci/variables/

Links to related issues and merge requests / references

#1809 (closed)

#26345 (closed)