FindCUDAToolkit: nvtx3 target

nvtx3 is a header-only replacement for the previous shared library implementations.

I implemented it as a separate target since while the header names match and ideally it should be API compatible, forcing its include directory into the old target would lengthen the include search path and could cause confusion or possible build differences for projects using multiple build systems. This keeps it explicit as a developer opt-in.

Implements: #21377 (closed)
Resolves: #23835 (closed)