Introduces mutation for instance model selection

Eduardo Bonetrequested to merge
565712-mutation-to-create-instance-level-settings into master

What does this MR do and why?

Introduces mutation for instance model selection

Extracts logic from Mutations::Ai::FeatureSettings into a service and adds logic to save instance level model selection when the provider is set to :vendored

Changelog: added EE: true

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Related to #565712 (closed)

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