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
https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2729/
## Success Criteria
1. [ ] Create `Start free trial` buttons on the GitLab.com Billing Page in the plan cards per the [designs](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2729). - FE Issue: https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2731
1. [ ] The buttons should be connected to the proper trial plan (Premium vs. Ultimate)
issue