Add endpoint to call verified namespace service

What does this MR do and why?

Parent issue

Adding an endpoint to run the VerifyNamespaceService. This endpoint will be run by the Gitlab customer support only.

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.

Screenshots or screen recordings

Screenshots or screen recordings

Before calling the endpoint:

Authorised:

When not a root namespace is passed & invalid verification level:

When a non root namespace is passed: When invalid verification level is passed: When non existant namespace path is passed: When caller of this endpoint does not have the credentials to do so: Non Gitlab.com instance

How to set up and validate locally

Follow steps described in !149532 (merged)
Run endpoint via http://127.0.0.1:3000/-/graphql-explorer

mutation {
  verifiedNamespaceCreate(input: { namespaceId: "root-level-group", 
    verificationLevel: "gitlab_maintained"
    }) {
    errors
  }
}