Set a Partial Response Flag on Incomplete Results

Continuing the discussion in [this](https://github.com/apache/pinot/issues/7264) ticket, it would be nice to have an explicit partial response flag whenever the results are incomplete. The earlier ticket adds new exceptions for partial results. However, it makes the fact that the result-set is incomplete a bit obfuscated from the user (he has to look into the exceptions list and then look for the error code). One more thing we can look at is failing the query if results are partial (making this behaviour configurable).

I created a new ticket for this as the earlier ticket, while it has much of the required discussion around this, had gotten a bit cluttered.

Start by reading the discussion in issue 7264 and tracing the existing exceptions and error-code handling for partial results. Define where the result-set’s incomplete state is exposed and how the configurable fail-on-partial behavior should work. Done means users can detect partial results through an explicit flag, with the configured failure behavior covered by tests.

Written by the indexing model from the issue text.