Draft: Add instance_level_third_party_ai_features_enabled to AppSettings (BE)

What does this MR do and why?

Add instance-level Application Setting to allow Third Party AI features for the entire Self-Managed instance.

FE-related (the checkbox) changes will be done in the next MRs.

For SaaS, we'll not render this setting, and will not have any effect, this is only for SM.

How to set up and validate locally

Switch to this branch

Run migrations: bin/rails db:migrate

In the Rails console, check that accessing the field does not throw an error:

[1] pry(main)> ::Gitlab::CurrentSettings.instance_level_third_party_ai_features_enabled
=> false

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #415546 (closed)