Add GraphQL endpoint to expose available group and instance templates

Problem

Backend part of #442350 (comment 1810798716)+.

We expose available list of instance and group templates via Rails controller. This approach doesn't support a pagination and should be replaced with a separate GraphQL endpoint.

Proposal

1. Add a GraphQL endpoint to fetch instance templates
2. Add a GraphQL endpoint to fetch group templates

An example of the response

{
  user(username:"user") {
    id
    groupTemplates {
      nodes {
        id
        name
        description
        
        projects {
          nodes {
            id
            name
            description
          }
        }
      }
    }
    instanceTemplates {
      nodes {
        id
        name
        description
      }
    }
  }
}