Update tracking for the Continuous Onboarding iteration 2 experiment

What does this MR do and why?

Adds some context & explicit stickiness to the change_continuous_onboarding_link_urls experiment & its tracking events.

Once we started collecting tracking data we realized that we’re not properly identifying different actors – all events are essentially being assigned to a single “user,” the anonymous user in Sisense. This MR resolves that.

Screenshots or screen recordings

Before

User ID	Namespace ID	Experiment Context Key
139	176	3dc0c3d3792025a8e19dd9a6c6028382
140	180	3dc0c3d3792025a8e19dd9a6c6028382

After

User ID	Namespace ID	Experiment Context Key
139	176	1a039de06fa717602882235b3ce3a9db
140	180	395f1df15ff2a2b5587854a990b03c37

How to set up and validate locally

1. Follow these instructions to get Snowplow Micro configured in your local gdk instance
2. Follow the general experiment setup instructions
3. Open a second browser window to the port you configured for your Snowplow Micro (default is http://localhost:9091/)
   • /micro/all to see an overview of all events collected so far (how many good, how many bad, etc.)
   • /micro/reset to reset the collected events to 0
   • /micro/good to see more details about the “good” events, specifically you’re looking for unique "data"."key" values in the gitlab_experiment context per user/namespace combo

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.

Related to #348469 (closed)