Multiple argument support for DISTINCTCOUNTHLL/DISTINCTCOUNTTHETASKETCH

Currently pinot only supports a single argument for distinct count hll and thetasketch functions. For some use cases like ours where we needed distinct count by two columns and `id` and a `date` it is very useful.

https://docs.pinot.apache.org/users/user-guide-query/how-to-handle-unique-counting

Start with the unique-counting documentation linked in the issue and trace the DISTINCTCOUNTHLL and DISTINCTCOUNTTHETASKETCH query-function entry points in Pinot. Done means both functions accept multiple column arguments and support distinct counting across combinations such as id and date, with the relevant behavior covered by the project's existing tests.

Written by the indexing model from the issue text.