Expose VSA stage event descriptions

What does this MR do?

This MR exposes the stage event (start, end) descriptions as HTML. For label based stages, we provide a custom description that includes the selected label nicely formatted (using Banzai).

Note: The change is not user facing, the FE will be implemented in #300235 (closed)

BE structure:

• Analytics::CylceAnalytics::GroupStage belongs to a Group and defines a time range by events: start_event, end_event.
• start_event, end_event are an instance of Gitlab::Analytics::CycleAnalytics::StageEvents::StageEvent, these are in memory objects
• StageEvent has a name. The name explains what the event is. It is used when selecting an event (dropdown)
• markdown_description field was added to StageEvent which falls back to name. The method is overridden in the label based event to provide a better description (includes the selected label).

Screenshots (strongly suggested)

Mockup:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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 to #300537 (closed)