OneTrust for trial and purchase flows pages

What does this MR do and why?

In https://gitlab.com/gitlab-org/gitlab/-/issues/341430 we're making the first iteration for the implementation of OneTrust. As a follow-up, we need to add this script to the trial and purchase flow pages.

Screenshots

Page	Console

How to set up and validate locally

Validate OneTrust not running

1. In rails console, make sure the feature flag is off

   Feature.disable(:ecomm_instrumentation)

2. Optionally add the one_trust_id config value to your gitlab.yml file

   ## OneTrust
   one_trust_id: '_your_one_trust_id'

3. Visit any of the listed pages above, such as http://127.0.0.1:3000/users/sign_in
4. Check that OneTrust is not installed/invoked. Running OneTrust in the Javascript console to get an undefined error or checking the network requests.

Validate OneTrust running properly

1. In rails console, make sure the feature flag is enabled

   Feature.enable(:ecomm_instrumentation)

2. Add the one_trust_id config value to your gitlab.yml file

   ## OneTrust
   one_trust_id: '7f944245-c5cd-4eed-a90e-dd955adfdd08-test'

3. Visit any of the listed pages above, such as http://localhost:3000/-/trials/new
4. Check that OneTrust is installed and running. Running OneTrust in the Javascript console to get an object or checking the network requests.

# 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.

Related to #342870