Log top-level relation creation when Importing Project/Group

What does this MR do?

This MR adds a log to every top-level relation creation when Importing a Project or a Group. This change is behind a feature flag and is intended to be used when importing into a group. This change is required for preparation to migration of users (with their groups & projects) from Githost to GitLab.com for observability purposes, so that it's clear which relation is created, and most importantly, which user has been assigned to it. More details in: &2623 (closed)

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