Add SecretsManager Provision Service

What does this MR do and why?

This MR adds a new LicenseAddOns and ProvisionServices class to enable the provisioning of the secrets_manager add-on.

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/597394+
• https://gitlab.com/groups/gitlab-org/-/work_items/21729+

Screenshots or screen recordings

N/A (backend changes only)

How to set up and validate locally

Please see this internal note for testing instructions

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #597394