Track gists import with snowplow events

What does this MR do and why?

Add Snowplow event to track successful and failed import of GitHub Gists into GitLab Snippet

Corresponding issue: #393163

Screenshots or screen recordings

When Gist was successfully imported

When it failed to import

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set up SnowPlow Micro on your gdk
2. Initiate a GitHub Gists import via POST {{host}}/api/v4/import/github/gists (Import GitHub gists into GitLab snippets for more details)
3. Go to http://gdk.test:9091/micro/all count should be changed
4. Go to http://gdk.test:9091/micro/good and check github_gist_import event to be present in the response with additional data

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.