Add BufUploader to implement same feature upon WriteMultipart like BufWriter

[Xuanwo]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

During implementing #5766, I find a missing piece of our current API set:

BufWriter adaptively uses put or put_multipart, depending on the volume of data written. However, there is no native implementation for PutPayload. This forces users to either use BufWriter, which requires an extra copy, or manually implement the adaptive logic.

Describe the solution you'd like

I plan to introduce BufUploader that fill this gap.

Describe alternatives you've considered

None.

Additional context

The PR has been sunmitted at #5835.

[alamb]

label_issue.py automatically added labels {'object-store'} from #5835