Export getter of values in BoundStatement.

There are some circumstances that we need to access (read-only) the bound values in the BoundStatement. For example, if you are writing a client wrapper of Session, and when query execution failed several times and you decide to try to reconnect the session (close the old session and create a new one), and if you try to execute the same bound statement using new session object, you'll get the DriverInternalError. In this case, you want to re-create prepare statement and bound statement from the bound statement object that caller gives you. Without this change, it's not possible to recreate the BoundStatement object.