Rename to Namespaces::AddOnPurchaseResolver

What does this MR do and why?

Rename to Namespaces::AddOnPurchaseResolver

Rename the resolver to GitlabSubscriptions::Namespaces::AddOnPurchaseResolver. This rename will convey better message that the query for add_on_purchase is scoped to namespace.

This raised from the previous MR discussion, and we wanted to create a separate small MR to keep the changes clean and readable.

How to set up and validate locally

All test passes on CI should be enough, as we are not changing any logic.

Additionally, we can follow this steps from the previous MR to test the existing functionality is working fine.

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 #432209