BE: Allow users to create a Premium trial - GitLab.com Billing Page Update

Summary

We've heard from customers via sales and support that there is a desire to offer Premium trials (currently only Ultimate are available) to allow users to truly validate whether the plan's features will meet the needs of their business.

Proposal

This issue is to allow users to select from options of Premium or Ultimate (SaaS or SM) trial.

Designs

#2729 (closed)

Success Criteria

1. Create Start free trial buttons on the GitLab.com Billing Page in the plan cards per the designs. - FE Issue: #2731 (moved)
   1. The buttons should be connected to the proper trial plan (Premium vs. Ultimate)
2. The main Start your free trial blue button at the top of the Billing Page should be connected to the legacy Ultimate trial plan #2731 (moved)