Add database structure for add-ons

db/docs/subscription_add_on_purchases.yml

db/docs/subscription_add_ons.yml

db/migrate/20230531134916_create_subscription_add_ons.rb

...te/20230531135001_create_subscription_add_on_purchases.rb

...ubscription_add_on_id_on_subscription_add_on_purchases.rb

...eign_key_namespace_id_on_subscription_add_on_purchases.rb

db/schema_migrations/20230531134916

db/schema_migrations/20230531135001

db/schema_migrations/20230531142032

db/schema_migrations/20230531142053

db/structure.sql

ee/app/models/ee/namespace.rb

ee/app/models/gitlab_subscriptions/add_on.rb

ee/app/models/gitlab_subscriptions/add_on_purchase.rb

ee/spec/factories/add_on.rb

ee/spec/factories/add_on_purchase.rb

ee/spec/models/ee/namespace_spec.rb

ee/spec/models/gitlab_subscriptions/add_on_purchase_spec.rb

ee/spec/models/gitlab_subscriptions/add_on_spec.rb