Add createMlModel mutation

What does this MR do and why?

Adds mutation to create ml models

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:model_registry)

2. open <gdk>/-/graphql-explorer. The following query should succeed (use a valid projectPath):

   mutation {
     createMlModel(input: { projectPath: "root/test_project", name: "awesome_name"}) {
       model {
         name
         id
       }
       errors
     }
   }

Related to #387580 (closed)