Log `notify_key` arg for import workers

What does this MR do and why?

This changes the import workers to log their notify_key arg.

#416306 (comment 1502279006)

How to set up and validate locally

1. Tail your sidekiq.log file: tail -f log/sidekiq.log.
2. In rails console:

   Gitlab::GithubImport::ImportDiffNoteWorker.perform_async(1, { one: 1 }, 'a_notify_key')

3. Your logs should record a worker queued with args: "args":["1","[FILTERED]","a_notify_key"].

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.