Create a separated query endpoint for multi-stage query engine

[xiangfu0]

This majorly beneficial for data Viz tooling like superset or retool which can specify a different query endpoint for multi-stage query engine without making any frontend change.

E.g. current superset connection string is: pinot://pinot-broker:8099/query/sql?controller=http://pinot-controller:9000/
Link to Add Pinot Database in Superset

We can create a new endpoint for multi-stage query at pinot://pinot-broker:8099/query/v2/sql?controller=http://pinot-controller:9000/

Changes proposed:

1. Add new query endpoint for multi-stage query engine in controller and broker
2. Follow up: Update java/python/golang clients to use the new endpoint.

[Jackie-Jiang]

I'm not a big fan of v2. If we are not going to change to v3 (hopefully not), maybe we can just use /query and eventually deprecate /query/sql