Add tracking events for trial reassurance experiment

What does this MR do?

Adds frontend tracking events (Snowplow) and records experiment subjects in our database for the Trial Registration Reassurance experiment.

Screencast (by means of Loom)

Trial Registration Reassurance experiment – tracking walkthrough - Watch Video on Loom

How to setup and validate locally

1. Follow these steps to install & enable Snowplow Micro in your local development environment
2. Make sure trials are enabled on your GitLab instance

   ApplicationSetting.first.update(check_namespace_plan: true)

3. Enable the experiment feature flag:

   Feature.enable(:trial_registration_with_reassurance)

4. It will be easier to test locally if you bypass these CustomersDot API response checks:

   # ee/app/services/gitlab_subscriptions/create_lead_service.rb, line 8
   if response.success || true
   
   # ee/app/services/gitlab_subscriptions/apply_trial_service.rb line 8
   if response.success || true

5. Navigate to /-/trial_registrations/new and begin the process
   • Check the /micro/good endpoint of Snowplow Micro to see the Structured Events (se types) in action
   • After the initial registration page submission, check the database for the ExperimentSubject record:

     es = ExperimentSubject.last
     es.experiment # make sure it's for the right experiment
     es.user # make sure the new user you created is here

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #335439 (closed)