Added event tracking to the merge request dashboard preferences dropdown

What does this MR do and why?

Added event tracking to the merge request dashboard preferences dropdown

References

#528953 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable merge_request_dashboard feature flag
2. Enable mr_dashboard_list_type_toggle feature flag
3. Follow this guide to track snowplow events: https://gitlab.com/gitlab-org/gitlab-foss/-/blob/v15.5.5/doc/development/snowplow/implementation.md#test-frontend-events
4. Visit merge request dashboard: /dashboard/merge_requests/
5. Click this button:
6. Select 'Show labels'
7. Observe two events to be tracked:
   1. open_display_preferences_dropdown_on_merge_request_homepage
   2. toggle_list_type_on_merge_request_homepage

MR acceptance checklist

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