Support in-memory segment generation and upload

Currently the SegmentWriter and SegmentUploader writes data into a TAR file and then upload to Pinot servers.

Can we optimize the write to disk by creating a in-memory TAR file format to speed up the upload?

Start by reading SegmentWriter and SegmentUploader to trace how data is written into a TAR file and uploaded to Pinot servers. Define the in-memory TAR approach and compare it with the current disk-based flow; done means segment generation and upload work without the intermediate disk write while preserving existing behavior.

Written by the indexing model from the issue text.