[FEA]: Allow `FutureValue` as the initial value passed to `DeviceReduce`

### 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.

I would like to pass asynchronous device results as the reduction initializer for `DeviceScan`

### Describe the solution you'd like

`DeviceScan` should accept `FutureValue`, or take an iterator as the reduction initializer.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_