feature request: support for preimage

[hartmut-co-uk]

As a consumer of my CDC event stream (Kafka topic), with table cdc preimages enabled, I'd like to also receive data of the preimage *_cdc_log record (cdc$operation=0).

This would allow me to fully utilise the change event for stream processing use cases.

Optional: either follow the cdc setting of the (source) table in question - or have the scylla-cdc-source-connector to explicitly configure (enable/disable) processing of preimages.

Example use cases:

• UPDATE to a table account where I'd like to determine if the col accountname has changed (+value from 'a' -> 'b')
• construct the full updated record (~postimage) to have a complete new record of my document, e.g. for streaming to other systems / databases