Toggle assignee and reviewer attention requested state

What does this MR do and why?

This change allows users to toggle the attention requested state on assignees and reviewers. The AttentionRequest service has been changed into an ToggleAttentionRequested service that will toggle between attention_requested and reviewed

Changes the GraphQL attention request mutation to be a toggle mutation which will then call the ToggleAttentionRequested service.

Finally this hooks it all up on the frontend so that the feature is functioning within merge requests.

Screenshots or screen recordings

Screen_Recording_2021-11-10_at_10.56.15

How to set up and validate locally

Enable the mr_attention_requests feature flag.

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 #343325 (closed)