Draft: POC for masked and hidden variables

What does this MR do and why?

Push the feature flag, expose :hidden, accept :hidden in patch Update settings section content if hidden variables are enabled Add hidden to graphql fragments, map masked and hidden attributes to visibility radio button options Disable radio buttons and value field if editing hidden variable Make the ci_hidden_variables feature flag use group actors

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

	Before	After
group variables		Screen_Recording_2024-07-18_at_11.11.33
project variables		Screen_Recording_2024-07-18_at_11.20.27
instance variables		no change

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #29674