Add sink config class to the manifest

[zzzming]

Currently we are missing sinkConfigClass specification.

{
"name": "cassandra-enhanced",
"description": "A DataStax Pulsar Sink to load records from Pulsar topics to Apache Cassandra(R) or DataStax Enterprise(DSE)\n",
"sourceClass": null,
"sinkClass": "com.datastax.oss.sink.pulsar.RecordCassandraSinkTask",
"sourceConfigClass": null,
"sinkConfigClass": null
}