Add CLOB support for Db2

Vert.x Db2 client currently [doesn't support type CLOB](https://vertx.io/docs/vertx-db2-client/java/#_db2_type_mapping):
```
Some types that are currently NOT supported are:

    XML

    BLOB

    CLOB

    DBCLOB

    GRAPHIC / VARGRAPHIC
```

We need to wait for https://github.com/eclipse-vertx/vertx-sql-client/issues/496 to be solved.