Add Duo Pro trial tracking events

What does this MR do and why?

Dedicated event tracking issue for the /-/trials/duo_pro/new (in-product) path for Duo Pro SaaS Trial

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

• Setup GDK to simulate SaaS.
• In rails console enable the experiment fully

  Feature.enable(:duo_pro_trials)

• Create new group.
• In rails console make it ultimate

  Group.last.gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'ultimate'))

• Visit group billing page. Click Start a Duo Pro free trial.
• Mare sure frontend event is generated.
• If you have snowplow micro setup locally, you can check other events as well.

Related to #435874