Add aiSelfHostedModelFeatureSettings query

What does this MR do and why?

Related to #476257 (closed)

In this MR, we:

• Add aiSelfHostedModelFeatureSettings query to fetch self-hosted model features

Screenshots or screen recordings

Success:

Errors:

How to set up and validate locally

• As an admin on a self-managed instance
• With self-hosted models enabled Feature.enable(:ai_custom_model)
• Perform the following query:

query {
  aiSelfHostedModelFeatureSettings(selfHostedModelId: "gid://gitlab/Ai::SelfHostedModel/$ID") {
    edges {
      node {
        feature
        provider
      }
    }
  }
}