Add onboarding steps for attention requests

What does this MR do and why?

Adds onboarding steps popover to the top navigation merge request badge and the attention request toggle in the merge request sidebar.

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the feature flag.

   Feature.enable(:mr_attention_requests)

2. Open the MR dropdown in the header.
3. Verify that the popover appears and click “Got it” to dismiss.
4. Repeat the two steps above to confirm that the popover was dismissed and doesn't appear again.
5. Visit an MR with at least one assignee or reviewer.
6. Verify that the popover appears and click “Got it” to dismiss.
7. Repeat the two steps above to confirm that the popover was dismissed and doesn't appear again.

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.

Closes #343329 (closed)