Add Instance Attribute to Service Model

With the assumption that we were going to deprecate Service Templates when introducing Instance-Level Integration Settings, !23595 (merged) was created to rename Template to Instance in the codebase.

However, with the change in plan (!23595 (merged)) to keep Service Templates functioning alongside Instance-Level Integration during a deprecation window, we want to keep the template attribute.

We can add the instance attribute to the table and a constraint that the service can either be:

an instance level service: project_id: nil, template: false, instance: true

a service template: project_id: nil, template: true, instance: false

a project service project_id: 123, template: false, instance: false

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