Refactor: add prepend mod to trail registrations controller

What does this MR do and why?

We are trying add a phone field to the trial registration form for Jihu Edition in this MR on jihulab.com. To override the allowed sign-up params, we should add the prepend mod to trial_registrations_controller.rb.

Our Proposal

1. add prepend mod to ee/app/controllers/trial_registrations_controller.rb
2. move the permitted param keys to a separated method sign_up_params_attributes
3. add specs to make sure the sign_up_params_attributes works properly

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

bundle exec rspec ee/spec/requests/trial_registrations_controller_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.