Improve Tracking for 4 experiments in onboarding flow

What does this MR do?

Record conversion for 3 experiment in onboarding flow

1. Addition to ee/app/controllers/trials_controller.rb, the trial onboarding experiment adds a new flow, where we should set tracking namely experiment_users.converted_at to some value for analytics purposes.
2. "For trial onboarding issues" we now record the namespace for the control group which remains in the trials_controller and we record the conversion

Followup for !52117 (diffs) https://gitlab.com/gitlab-org/gitlab/-/issues/276703#note_502956661

This data (experiment.users.context, experiment_users.converted_at and other fields) are used on Sisense charts by the growth teams.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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