Customizable Cycle Analytics - Add stage button

What does this MR do?

Implement add stage button for adding customisable stages to cycle analytics. This is a ~"GitLab Enterprise Edition" only feature.

• Implement the Add stage button
• Adds async imports to load the components into the existing cycle analytics page
• Adds a mixin for the required Add a stage functionality
• Adds a stub mixin to CE

active = false	active = true

Feature flag

:customizable_cycle_analytics

Related Issues

https://gitlab.com/gitlab-org/gitlab-ee/issues/13076

CE Backport

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31791

Does this MR meet the acceptance criteria?

Conformity

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

Performance 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

Closes #13076 (closed)