Add snowplow custom emitter

What does this MR do and why?

Add a custom async emitter to debug #562794

Code changes summary:

• Creating a new custom emitter feature flag configuration that's disabled by default
• Modifying the tracking system to use a custom asynchronous emitter when the flag is enabled. This emitter uses Gitlab's sidekiq for processing background jobs instead of snowplow's default system that spins up thread. This is done so we can be able to monitor the background processes.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable snowplow_job_emitter by running: Feature.enable(:snowplow_job_emitter)
2. Trigger an event via the rails console or the UI,
3. Start the snowplow-micro and ensure that events are getting triggered.

MR acceptance checklist

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