Store started_at when received from customers portal

What does this MR do and why?

The recent change to extend Duo Pro trials uncovered that we don't store the start date of Duo Pro trials. With the [data storage in place(!152857 (merged)) this MR updates the /api/v4/namespaces/#{namespace_id}/subscription_add_on_purchase/#{add_on_name} endpoints to accept a new started_on parameter sent from the Customers Portal application, and store that value in the started_at column of the subscription_add_on_purchases table.

Related Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/459962

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

1. Boot GDK with GITLAB_SIMULATE_SAAS=1 set
2. wait for GitLab to boot
3. Define GDK_INSTANCE, NAMESPACE_ID, ADMIN_TOKEN in your shell, where GDK_INSTANCE is the base url of your GDK instance, NAMESPACE_ID is the id of a namespace that exists within your GitLab instance and has Duo Pro seats, and ADMIN_TOKEN is an access token valid for the root user of your GDK instance.
4. Update an add on associated with that namespace with

curl http://$GDK_INSTANCE/api/v4/namespaces/$NAMESPACE_ID/subscription_add_on_purchase/code_suggestions -X PUT -d "{\"quantity\":5,\"started_on\":\"2024-05-24\",\"expires_on\":\"2025-06-24\",\"purchase_xid\":\"A-12345678\",\"trial\":false}" -H "Accept: application/json" -H "Content-Type: application/json" -H "PRIVATE-TOKEN: $ADMIN_TOKEN"

5. Note that started_at is included in the response
6. Fetch the add on data with

curl http://$GDK_INSTANCE/api/v4/namespaces/$NAMESPACE_ID/subscription_add_on_purchase/code_suggestions -H "Accept: application/json" -H "Content-Type: application/json" -H "PRIVATE-TOKEN: $ADMIN_TOKEN"

7. See that the started_at is present and matches what was previously set.