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

[pciolkosz]

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

[jrhemstad]

Why does the concept require copyability? Not all resource implementations will be copyable.

[pciolkosz]

Might be a requirement of the any_[async_]resource?

@ericniebler might have a better answer here