PEP: Redesign multi topic realtime ingestion support

Multitopic realtime ingestion as it currently exists has some issues. In this [PR](https://github.com/apache/pinot/pull/18433), @Jackie-Jiang said

The multi-topic partition id overloading existing partition id has caused so many issues:

We are missing checks here and there
Stream with custom partition id fails (See https://github.com/apache/pinot/issues/17260)
I think it is the time to revisit this design decision and avoid the troubles going forward. I can see 2 possible solutions:

Make partition_id part within the table name x_y where x is the topic id and y is the partition id. For single topic name, it remains the same
Leverage logical table for multiple topics, each topic a separate physical table

This is the PEP request to make those changes. I've detailed both suggestions in this doc:
https://docs.google.com/document/d/1dY6ZvaG_-FbTAQ5e1lxB4uvi8VGbR_WfCfCl5zD76A4/

Start by reading the linked Google design document and PR #18433, including the cited issue about custom partition IDs. Compare the two proposed redesigns and determine the required scope; the issue names no files, tests, or concrete completion criteria, so implementation readiness is not established.

Written by the indexing model from the issue text.