Allow creating a NinjaAPI with no urls namespace

[anentropic]

I'm trying to incrementally migrate some Django views to Ninja API views

I want my un-migrated Django views to exist in the same url namespace as the migrated Ninja API views (i.e. for the migrated views to have exactly the same url name as the view they are replacing)

It seems this is currently not possible, because Ninja API always has its own namespace, so it's not possible to add the API urls un-namespaced to an existing namespace

I realise namespacing is a good default, but there are cases like mine where it would be useful to be able to go outside that default (avoiding url name clashes would be my responsibility)