Introduce class to easier process license add-on purchases

What does this MR do and why?

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/465546+

⚠ Depends on Refactor license add-on provisioning (!164457 - merged) to be merged first

This MR was created to help split a bigger MR/change into smaller pieces.

Currently, only the quantity of a license add-on purchase is used. With incoming changes, more information of a license add-on purchase will be processed and stored.

This change introduces a new class to easily process a license add-on purchase. In addition, the mocked license add-on products in various specs are extended to include additional info.

MR acceptance checklist

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

How to set up and validate locally

A green test suite should be sufficient for this change.