[CUDAX] Make [async_]resource_ref assignable with rebind semantics

Currently `__iasync_resource` interface requires `icopyable`, which lowers to `uda::std::copyable`, which contains `requires(assignable_from<_Tp&, _Tp&>)`.

`[async_]resource_ref` is `basic_any` containing a reference type and to model references it deletes assignment. In order to make `[async_]resource_ref` to be properly recognized as a resource we should add assignment with rebind semantics to the type