Add more comprehensible logs to FIELD event

The FIELD event contains the table definition. VStream doesn't send the FIELD event with every ROW event. VStream only sends the FIELD event when necessary (e.g. when restarting the connector, or after the table's definition has been changed). So its log is not verbose, compared to INSERT/UPDATE/DELETE events.

When DEBUG Level logging is configured, having more comprehensible logs for the FIELD event would help debugging ROW event's columns and in-memory table schema's columns mismatch, should this occasion arise.