Implement hardware selector in webgpu

- This commit implements the features for hardware selector in vtkWebGPUHardwareSelector.
- It implements the superclass `GetPixelInformation()` function to parse the contents of the Ids attachment into items of type `vtkHardwareSelector::PixelInformation`.
- The new `TestHardwareSelector.cxx` exercises picking geometry in a single pixel and a rectangle of pixels using webgpu backend.