Support dynamic updates to existing derived columns in table

Currently Pinot doesn't support runtime changes to existing derived columns (for e.g change to ingestion transform expression) in table. Only way to make such updates is to either delete/recreate the table and ingest from scratch or add a new derived column with the changes, neither of which are ideal from usability perspective.

Can we consider improving this behavior so users can easily make adjustments to the data via table config/schema change and subsequently doing a segment reload ? cc @Jackie-Jiang @mayankshriv

Start by tracing Pinot's table config/schema update path and the segment reload workflow, since the issue names no specific files or tests. Define completion as allowing an existing derived column's ingestion transform expression to change through configuration or schema updates, followed by a segment reload, without recreating the table or ingesting everything from scratch.

Written by the indexing model from the issue text.