Implement callproc() for full DB-API 2.0 compliance

Summary

callproc() currently raises NotSupportedError. It is the only remaining gap to full PEP 249 (DB-API 2.0) cursor interface compliance.

Current behavior

cursor.callproc('dbo.my_proc', [1, 'abc'])
# Raises: NotSupportedError

Workaround

Users must use cursor.execute() with either EXEC or the ODBC {CALL ...} escape sequence:

cursor.execute('{CALL dbo.my_proc(?, ?)}', [1, 'abc'])

Expected behavior

callproc() should execute a stored procedure and return modified copies of the input parameters, per the PEP 249 specification.

Impact

The driver currently reports apilevel = '2.0' and the cursor interface is listed as "Mostly compliant" solely because of this gap.