[FEA]: Support device-resident problem size in device scan

### 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

CUB

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

Currently, `cub::DeviceScan` accepts `num_items` as an integer. Sometimes, problem size is not available at host side.

### Describe the solution you'd like

This issue can be closed with a PR adding device-resident problem size support in device scan:

int *num_items = ...;
cub::DeviceScan(..., cuda::args::deferred{num_items})
Test should include passing a fancy (say, transform) iterator into deferred.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_