[FEA]: Provide `mdspan` padded layouts

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this request and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Area

libcu++

### Is your feature request related to a problem? Please describe.

`mdspan` padded layouts are essential to efficiently perform computation on matrices with padding and to select sub matrices with compile-time dimensions

### Describe the solution you'd like

Provide C++26 `mdspan` padded layouts and backport to C++14/17:

- `cuda::std::layout_left_padded`,
- `cuda::std::layout_right_padded`

### Describe alternatives you've considered

_No response_

### Additional context

_No response_