Add snowplow event tracking to group imports

What does this MR do and why?

Describe in detail what your merge request does and why.

This change adds snowplow tracking events for group imports that use either the bulk import or file import features. This is something requested as useful for product to gain more insight into monthly user GitLab Group migration flow.

Tests have been added that use the snowplow helper method as per the best practices guide

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #357258 (closed)