Add opt-in user preference toggle for Rapid Diffs in merge requests

Summary

Implement a user-facing toggle that allows users to opt-in to Rapid Diffs for MRs.

Requirements

• User preference toggle in the interface to opt-in to the new experience
• Default to existing implementation (opt-in only)
• Toggle state persists across sessions
• Link to feedback issue (comment)
• Ability to toggle back to current experience
• Tracking of enabled (comment)

Related

Part of the rollout strategy for &18457.

Designs

Figma

Default	Hover to show popover	Enabled - Dropdown

1. When Rapid Diffs enabled = false (default)
   • Add button style confirm/tertiary with label "Try Rapid Diffs"
   • When hover over button, display popover
     • Title: Improved performance loading diffs
     • Body: Rapid Diffs is a feature enhancement designed to speed up the loading and interaction with code changes.
   • When clicked then rapid diffs is enabled
     • Track rapid diffs enabled
2. When Rapid Diffs enabled = true
   • Display badge in neutral style with label "Rapid Diffs enabled"
   • When click, display dropdown menu
     • Option 1
       • Label: "Leave feedback"
       • Icon: comment-dots (correction: this change is not reflected in screenshots and video)
       • Click navigates to feedback issue (tbd)
     • Option 2
       • Label: "Switch to classic loading"
       • Icon: substitute
       • Click set Rapid Diffs as no longer enabled
         • Decrement count for rapid diffs enabled

Other notes/considerations

1. We have precedence of showing dropdowns on badge clicks When visiting a file (blob) page provide a way ... (#448868 - closed)
2. Did consider using a toggle button but did not think that the toggle button looked nice (Figma).
3. Did consider putting it at the top next to Edit and Code buttons for better discovery but decided to keep it closer to the diffs which is in the "Changes" tab (Figma).