Use a two server setup for multi-stage query engine backward compatibility regression test suite

[yashmayya]

• Add backward compatibility regression test suite for multi-stage query engine #13193 added a backward compatibility regression test suite for the multi-stage query engine, leveraging the framework added in Framework for adding compatibility tests #6129, complete compatibility regression testing #6650, Create test suite across pinot releases and services #4001.
• However, there was one major element missing - the framework relies on a single controller, broker and server setup for Pinot. While this was fine for the v1 query engine backward compatibility tests, using a multi server setup for the multi-stage query engine backward compatibility tests is much more important. Since v2 queries typically involve data shuffling between servers, we also need to test scenarios where one server is running on an older version and another is on a newer version.
• This patch adds an optional second server to the generic framework which is leveraged by the multi-stage query engine backward compatibility regression test suite. Since the segments are added sequentially in the test framework, and the default balanced segment assignment strategy is used (https://docs.pinot.apache.org/operators/operating-pinot/segment-assignment), we know that the queries involving joins between the two tables will result in some data shuffling.
• This isn't easily extensible for adding more controllers / brokers / servers which could've been done via some form of parameterized configuration; however, doing so would likely break existing config file support (Add config file support, update yaml files with correct data for Compatibility Testing, fix generation support for queries(#6787) #6901) and currently it seems sufficient to have a 1 controller / 1 broker / 2 server setup for testing backward compatibility.