First iteration for making Duo Pro in-app trials 1 step

What does this MR do and why?

First iteration for making Duo Pro in-app trials 1 step. Introduce new in-app trial form for Duo Pro with only lead parts

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK

2. Create new group.

3. In rails console make it premium.

   bin/rails c
   
   Group.last.gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'premium'))

4. In rails console enable the feature flag

   Feature.enable(:duo_pro_trial_single_form)

5. Visit /-/trials/duo_pro/new path.

MR acceptance checklist

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

Related to #543765