Backport of 'Add paidTierTrial to subscriptionUsage GraphQL API'

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/customers-gitlab-com/-/work_items/15815

See merge request !221784 (merged)

This addition was originally added in Add paidTierTrial to subscriptionUsage GraphQL API (!221784 - merged) to support paid tier trials (bonus trials) for GitLab Credits. This is being backported to 18.8 in order to support existing customers on this version who want to take advantage of trials for the upcoming Transcend event on Feb 10.

Please see https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/15806+ for more details.

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
  • This is isn't a bug fix but a high priority feature necessary to support the upcoming transcend event. Special approval was given to backport these updated to 18.8.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.