Configure Snowplow Micro using yml

What does this MR do and why?

Related to #358623 (closed)

We want to configure Snowplow Micro without using ENV variables, to make it work more intuitively. Instead, we will be using an already existing GDK setting, exposed to us with this MR.

Screenshots or screen recordings

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How to set up and validate locally

1. Have snowplow micro running locally
2. Checkout this MR's branch for gitlab and this branch for the GDK.
3. Run gdk reconfigure
4. Open the rails console and try to trigger an event. You can use this snippet to do it: Gitlab::Tracking.event('Projects::TestCategory', 'test_action')
5. Make sure that the event has been collected at the localhost:9091/micro/good endpoint.
6. Try to change the snowplow_micro.port value using the gdk config command and repeat the steps 3-5 to make sure it's still working.

MR acceptance checklist

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