Multi-stage query engine doesn't respect the enable.case.insensitive cluster config

[yashmayya]

• Currently, the multi-stage query engine always treats identifiers as case sensitive.
• The single-stage query engine defaults to treating identifiers as case insensitive but allows case sensitive behavior via the cluster config enable.case.insensitive.
• Postgres and most other common databases treat identifiers as case insensitive (unless they are explicitly wrapped in double quotes) and we should try to do the same.
• This difference is currently documented (https://docs.pinot.apache.org/reference/troubleshooting/troubleshoot-multi-stage-query-engine#table-names-and-column-names-are-case-sensitive) but it would be good to remove the restriction and make v1 -> v2 migrations easier.