Adds GraphQL action to delete models

What does this MR do and why?

Adds a new graphql action to delete ml models.

How to set up and validate locally

1. In rails console enable the experiment fully, and create a model

   Feature.enable(:model_registry)
   p = Project.find_by(id: 1)
   Ml::FindOrCreateModelService.new(p, "model_1").execute 

2. Navigate to <gdk>/-/graphql-explorer, and execute the following mutation (replace values with your project and model id):

   mutation {
     mlModelDestroy(input: { projectPath: "root/test-project", id: "gid://gitlab/Ml::Model/1" }){
       errors
     }
   }
   

3. Model should be deleted without returning errors

Related to #428910 (closed)