Paginate discussions endpoint

What does this MR do and why?

Add pagination for the discussions endpoint. This loads the discussions in batches for issues.

Sample queries

1. 1st page: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7208/commands/25491
2. 2nd page: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7208/commands/25493

How to set up and validate locally

1. Enable the paginated_issue_discussions feature flag
2. Visit an issue with lots of discussions
3. See that the notes are loaded in batches. You can also verify by searching for discussions.json requests in the Network tab.

paginated_issues

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.

Related to #340172 (closed)