Add Snowplow buffering

What does this MR do and why?

Related to #508460 (closed), #517359 (closed)

We want to test out buffer sizes with Snowplow.

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Before	After

How to set up and validate locally

1. Run Snowplow Micro locally: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/snowplow_micro.md
2. Enable the feature flags: Feature.enable(:snowplow_tracking_post_method); Feature.enable(:snowplow_buffer_events)
3. Restart the rails console. This is to make sure the Snowplow class instance gets initiated again.
4. Trigger an event a few (less than 100) times, for example by running this in the rails console: 3.times { Gitlab::InternalEvents.track_event('use_run_keyword_in_cicd_yaml', user: User.first) }
5. Make sure that the event have NOT been recorded by Snowplow Micro: http://127.0.0.1:9091/micro/ui
6. Trigger the events again, this time making sure you've passed 10 events: 10.times { Gitlab::InternalEvents.track_event('use_run_keyword_in_cicd_yaml', user: User.first) }
7. Make sure that the a batch of 100 events has been recorded by Snowplow Micro: http://127.0.0.1:9091/micro/ui
8. Now, test the at_exit hook: make sure that you have triggered some events that have not been sent yet [eg. when following instructions supplied in the previous steps, you should have 3 batched unsent events]. Then, close the console, eg. by running exit or ctrl+d.
9. Make sure that the leftover events have been sent to Snowplow: http://127.0.0.1:9091/micro/ui