Support for Tenant data isolation

We have seen requirements where multiple tenants (clients) need to be hosted on the same Pinot table (one table per client is not scalable). However, the tenants would like to ensure that their data is isolated from other tenants' data on the Pinot table. Isolation here means:

- Guarantee that a client's (tenant) data is protected against any leaks (accidental or otherwise) to other clients.
- Certain use cases may also require tenant/client specific encryption keys to further ensure data isolation

(Note tenant and client are used here interchangeably).

No files, tests, or entry points are identified in the issue. Start by reviewing Pinot's table storage, query isolation, and encryption architecture, then clarify the required isolation guarantees and tenant-specific key handling with maintainers. Done means an agreed design and implementation plan that prevents cross-tenant data exposure.

Written by the indexing model from the issue text.