Introduce the Gitlab::Insights framework

What does this MR do?

This introduces the Gitlab::Insights framework which goal is to find issuables, then reduce them into graph-compatible data and serialize this data into a charting library-compatible format.

What are the relevant issue numbers?

This is a preliminary to https://gitlab.com/gitlab-org/gitlab-ee/issues/9867 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9776.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• 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
• Security reports checked/validated by reviewer