Resolve "Add median line to cycle analytics duration chart"

What does this MR do?

This MR adds the median line to the cycle analytics duration chart.

We fetch a second set of duration chart data but offset by the number of days that is being displayed. i.e. if 30 days of data is being displayed, we'll fetch the 30 days of data prior to the date range being displayed.

Fetching the additional data will happen in a separate series of API requests in order to display the chart asap.

Adding a 6 month range limit will be addressed in a follow up issue #119111 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Steps taken to make reviewing this easier

• Chart merged here: !14996 (merged)
• Date util methods merged here: !22671 (merged)
• Cycle analytics utils methods merged here: !22802 (merged)

Conformity

• [-] Changelog entry
• Documentation MR !23062 (merged)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related Issue

#36675 (closed)