Add support for lazy loading Epic children within tree

What does this MR do?

Note: This MR adds a feature to Epic Tree (added in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10999) which is currently behind feature flag :epic_tree.

This MR enables lazy loading additional Epic children when the children count is over 100 as GraphQL currently limits single page to have not more than 100 items.

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Closes #11971 (closed)