GH-47713: [C++][FlightRPC] ODBC return number of result columns (#48036)

### Rationale for this change
Implement ODBC to return the number of result columns

### What changes are included in this PR?
- SQLNumResultCols implementation & tests
### Are these changes tested?
Tested locally on MSVC
### Are there any user-facing changes?

 N/A

* GitHub Issue: #47713

Authored-by: Alina (Xi) Li <alina.li@improving.com>
Signed-off-by: David Li <li.davidm96@gmail.com>