Add masked parameter to group variable api

What does this MR do?

• Expose parameter masked onto group variables api.
• Add documentation about parameter masked onto group variables api.
• Add missing field protected into documentation https://docs.gitlab.com/ee/api/group_level_variables.html.

I want to use Terraform to manage group variables. Unfortunately, This feature is not available because masked parameter is not exposed via API.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team