Maps featureSettings in SelfHostedModelType

What does this MR do and why?

Maps featureSettings in SelfHostedModelType

The feature settings an AI model is assigned to is now accessible when querying self-hosted models in graphql.

Steps to reproduce

1. On rails console:

Ai::SelfHostedModel.create()

2. navigate to http://gdk.test:3000/-/graphql_explorer, and run the following query:

   query getSelfHostedModels {
     aiSelfHostedModels(first: 100) {
       nodes {
         id
         name
         model
         endpoint
         hasApiToken
         featureSettings {
           nodes {
             feature
           }
         }
       }
     }
   }   

   The result should be something in the format:

   {
     "data": {
       "aiSelfHostedModels": {
         "nodes": [
           {
             "id": "gid://gitlab/Ai::SelfHostedModel/20",
             "name": "a_cool_model",
             "model": "mistral",
             "endpoint": "http://localhost:4000",
             "hasApiToken": false,
             "featureSettings": {
               "nodes": [
                 {
                   "feature": "code_generations"
                 }
               ]
             }
           }
         ]
       }
     }
   }

Related to #476258 (closed)