Add trigger indexing worker

What does this MR do and why?

Related to #434590 (closed)

Behind a feature flag

Makes the gitlab:elastic:index rake task asynchronous through a new worker

Screenshots or screen recordings

N/A

How to set up and validate locally

1. setup gdk for elasticsearch
2. start elasticsearch

     gdk start elasticsearch

3. index everything, this will use the old method

     bundle exec rake gitlab:elastic:index

4. Go to Admin Area > Settings > Advanced Search to enable Elasticsearch.
   1. Elasticsearch indexing and Search with Elasticsearch enabled should be checked
5. confirm search works and is using Advanced search (should see a message: Advanced search is enabled)
6. enable the feature flag

      echo "Feature.enable(:elastic_index_use_trigger_indexing)" | gdk rails c

7. re-run the rake task

     bundle exec rake gitlab:elastic:index

8. confirm the worker runs successfully in the sidekiq logs
9. can also check status in **Admin Area > Settings > Advanced Search > Indexing status
10. confirm search works and is using Advanced search (should see a message: Advanced search is enabled)

MR acceptance checklist

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