CassandraSinkTask throws an Exception if the SinkRecord has a field of STRUCT type

[aaruna]

[aaruna]

This is partly fixed by #18
So when cassandra.sink.field.mapping is set, then there is no Exception thrown.
However, if that property is not set, we assume that there is a one-to-one mapping from Fields to the Columns, so we get an error if there is a field of STRUCT type.
We can avoid this by ignoring unsupported CQL types, rather than trying to stringify the field value and using the same in INSERT statement.