Add support for jsonb type

[edulauer]

When a PostgreSQL source table contains a column of type jsonb, using DbtoDbOperator to copy data into a SQL Server table results in the following error:

Error message:

   destination_cur.executemany(insert, rows)
pyodbc.ProgrammingError: ('Unknown object type dict during describe', 'HY000')

Possibly workaround:
Before calling executemany, convert any dict values to JSON strings using json.dumps()