Add support for event creation on designs

What does this MR do?

This MR is Part of #33051 (closed)

This MR builds on !32700 (merged) by adding service level support for creating events on designs. New methods are added to the EventCreateService and hooks are adding in the existing design services (save and delete) to create events on success.

Feature Flag

This work is enabled behind a feature flag, gated by project. To enable this feature, in your rails console:

Feature.enable(:design_activity_events, project)

This feature should not be enabled until the views have been updated so that design events can be viewed correctly.

Screenshots

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