Add BEGIN/END, BEGIN..WITH support

Per Jeremy @refset You can set the system time of a transaction as part of the BEGIN statement - BEGIN READ WRITE WITH (system_time = TIMESTAMP '...') (or BEGIN READ WRITE WITH (system_time = ?) if you want to pass it as a parameter). <!-- Edit the body of your new issue then click the ✓ "Create Issue" button in the top right of the editor. The first line will be the issue title. Assignees and Labels follow after a blank line. Leave an empty line before beginning the body of the issue. -->