feat: manual variables tab frontend part two

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What does this MR do and why?

• Introduces Feature Flag for manual variables.
• Introduces UI for manual variables under pipeline details.

Notice

Right now, role including and under Developer will end up white screen due to issue #323097 (closed), and there is a fix !158314 (merged), make sure you visit this tab with role above Developer

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

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State	Screenshot
Empty
Maintainer Role with data concealed (15+ data)
Maintainer Role with data revealed (15+ data)
Maintainer Role with data concealed (< 15 data)
Developer Role (15+ data)

Screencast for new badge dismissible behavior

CleanShot_2024-07-19_at_15.47.16

How to set up and validate locally

1. Checkout to the newest commit.
2. Use terminal to boot Rails Console via command rails c.
3. Enable the feature flag by Feature.enable(: ci_show_manual_variables_in_pipeline).
4. Go to any projects with pipeline configured (with maintainer role).
5. Create a manual pipeline with manual variables.
6. Go to the triggered pipeline detail page and click on the Manual Variables tab
7. Observe.

• Switch account to a developer role repeat step 2-5.

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