Support partitioning on composite column

The current mechanism (https://docs.pinot.apache.org/operators/operating-pinot/tuning/routing#partitioning) allows specifying the partitioning logic on a single column. There may be cases where we want to partition the data on a composite column (eg: ). If the data is already partitioned on a composite column - we should have a way of capturing that in the Pinot config

Start with the partitioning documentation at the linked routing page and trace how Pinot currently captures single-column partitioning in its configuration. Define how a composite column such as should be represented and verified, then confirm the configuration supports data already partitioned that way.

Written by the indexing model from the issue text.