[C++] Fix TODO error regarding complex type on `GetTablesReader::GetSchema()` in Arrow Flight SQL ODBC

[alinaliBQ]

Describe the enhancement requested

Fix the error indicated by TODO statement in flight_sql_get_tables_reader.cc. The driver needs to be tested with an Arrow Flight SQL server.
After this TODO is fixed, remove TODO statement in flight_sql_statement_get_columns.cc

Component(s)

FlightRPC, C++