Audit events for update of self-hosted models

What does this MR do and why?

• Refactors update of SelfHostedModels to a service
• Audits update to self-hosted model configurations

How to set up and validate locally

1. on gdk rails c, execute the service:

   model = Ai::SelfHostedModel.create(name: "my model 3", model: "codestral", endpoint: "http://localhost.com")
   Ai::SelfHostedModels::UpdateService.new(model, User.find_by_id(1), { name: 'a new model' }).execute

2. Still in the console check the last audit event and see if information is correct

   AuditEvent.last

Related to #483295 (closed)