Added attention required icon to assignees and reviewers

What does this MR do and why?

Adds a toggle to assignees and reviewers that will toggle the attention required state. Note for now this does not call the API as that is in a different merge request.

Closes #343326 (closed)

Screenshots or screen recordings

How to set up and validate locally

Enable the mr_attention_requests feature flag.

MR acceptance checklist

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