Add duo_pro_trials_for_free_plans feature flag

What does this MR do and why?

Create duo_pro_trials_for_free_plans feature flag to control Duo Pro trials for free tier namespaces on Gitlab.com

MR acceptance checklist

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free	paid

How to set up and validate locally

• Setup GDK to simulate SaaS.

  bin/rails c
  
  Feature.disable(:duo_pro_trials)
  Feature.disable(:duo_pro_trials_for_free_plans)

• Sign in. Create new group on free plan.

• Visit billing page.

• Make sure there is no Start a free GitLab Duo Pro trial button.

  bin/rails c
  
  Feature.enable(:duo_pro_trials)

• Make sure there is no Start a free GitLab Duo Pro trial button.

  bin/rails c
  
  Feature.disable(:duo_pro_trials)
  Feature.enable(:duo_pro_trials_for_free_plans)

• Make sure there is no Start a free GitLab Duo Pro trial button.

  bin/rails c
  
  Feature.enable(:duo_pro_trials)
  Feature.enable(:duo_pro_trials_for_free_plans)

• Make sure there is Start a free GitLab Duo Pro trial button.

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

• Make sure there is Start a free GitLab Duo Pro trial button.

  bin/rails c
  
  Feature.disable(:duo_pro_trials_for_free_plans)

• Make sure there is Start a free GitLab Duo Pro trial button.

  bin/rails c
  
  Feature.disable(:duo_pro_trials)

• Make sure there is no Start a free GitLab Duo Pro trial button.

Related to #443734