Fetch epics as Roadmap is scrolled and timeframe is expanded

What does this MR do?

This MR is follow-up on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9018 and adds networking support to pull in new epics as timeline is scrolled.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

#7325 (closed)