Add additional_properties to UsageData API

What does this MR do and why?

Related to #434504 (closed)

With !142839 (merged), we made it possible to handle additional_properties attribute on the backend.

Now, we want to make it possible to pass it from the frontend using our UsageData API.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Enable snowplow micro
2. Choose an internal event and read its RedisHLL counter, for example: Gitlab::UsageDataCounters::HLLRedisCounter.unique_events(event_names: 'user_viewed_dashboard_list', start_date: Date.yesterday, property_name: :user, end_date: Date.tomorrow + 7.days)
3. Create a personal access token
4. Trigger the affected endpoint while sending additional_properties, eg. curl 'http://127.0.0.1:3000/api/v4/usage_data/track_event?private_token=PRIVATE_ACCESS_TOKEN' -H 'Content-Type: application/json' --data-raw '{"event":"user_viewed_dashboard_list","namespace_id":1,"send_snowplow_event":true,"additional_properties":{"label":"label123"}}', replacing PRIVATE_ACCESS_TOKEN with the created token
5. Check the RedisHLL counter again [eg. Gitlab::UsageDataCounters::HLLRedisCounter.unique_events(event_names: 'user_viewed_dashboard_list', start_date: Date.yesterday, property_name: :user, end_date: Date.tomorrow + 7.days)]. This should return a value increased by 1 compared to the last time.