Extend load generator to use flightsql client for querying

Existing load generator already supports query command but under the hood it uses HTTP client, in order to compare the performance of flightsql client with HTTP client, extend the query command to support `--query-type` that takes in `http` or `flight` as parameters.

The existing load generator's query command is the entry point; start by tracing its current HTTP-client path. Add support for --query-type with http and flight values, and verify that each selects the corresponding client so the generator can compare performance.

Written by the indexing model from the issue text.