Add hide and reveal button for variable values

What does this MR do and why?

While working on the removal of the FF !128913 (merged) I noticed the missing feature to hide/show variable values.

This MR adds that ability back into the pipeline schedules refactor to not disrupt our customers.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Example below:

1. In rails console enable the experiment fully

   Feature.enable(:pipeline_schedules_vue)

2. Visit a pipeline schedule and add variables and test button.
3. Create a pipeline schedule

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.