Convert IID fields from ID to String types

What does this MR do and why?

This MR changes IID fields that use the GraphQL ID type to be String type instead, following our guidance on the use of ID types !150604 (merged).

In a follow-up MR a cop will help enforce this !152003.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

On both master and this branch, there will be no change in how the fields are received:

For example, in GraphiQL http://gdk.test:3000/-/graphql-explorer:

{
  issue(id: "gid://gitlab/Issue/10") {
    iid 
  }
}