Partial update in Pinot

Issue https://github.com/apache/incubator-pinot/issues/4261#event-3951115974 was closed after the full update is supported. There are scenarios that the ingested message is for the partial update. The partial update could be:
- Only the fields updated have the value, while other fields are ignored
- The update fields may contain the incremental value, but not the updated value. e.g. trip fair +$5

Start by reviewing the closed issue 4261 and the existing full-update support mentioned there. Determine where ingestion and update handling are implemented, then define and test support for updates containing only changed fields and for incremental values such as trip fare increases.

Written by the indexing model from the issue text.