Cycle Analytics - Duration chart / scatter plot

What does this MR do?

This MR adds the duration chart scatterplot required for the new cycle analytics feature

Does this MR meet the acceptance criteria?

Conformity

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

Related issue

https://gitlab.com/gitlab-org/gitlab-ee/issues/12183

Todo

• Stage dropdown should have all options selected when expanded - !20446 (merged)
• Duration data should be fetched once, after cycle analytics data
• Duration data needs to be transformed
• Duration chart should be filtered from state data on stage selection
• Follow up issue for legend - on the discrete scatter component
• Add feature flag
• Add message for when no data is returned
• Fix chart tooltip bug
• Tests need to be added / updated
• Follow up issue for median line - #36675 (closed)
• Refactor endpoint based on !20192 (merged)

Testing locally

• Ensure that you have a premium+ licence installed
• Ensure that the following 2 feature flags are enabled
  • cycle_analytics
  • cycle_analytics_scatterplot_enabled
• Pick commits from !19293 (merged) if it hasn't been merged

Seeding cycle analytics data

SEED_CYCLE_ANALYTICS=true bundle exec rake db:seed_fu FILTER=cycle_analytics

Breaking this MR down

• Stage filter dropdown changes !20446 (merged)
• Adding duration_chart endpoint to the frontend API !20563 (merged)
• Adding the required utils methods !20566 (merged)