[rocketmq-connect] format converter

1. Please describe the issue you observed:
- The converter logic is integrated with sendRecord, which is not conducive to subsequent expansion.

2. Other information (e.g. detailed explanation, logs, related issues, suggestions how to fix, etc):
- Abstract the converter logic into the corresponding Converter object.

Start by locating the rocketmq-connect sendRecord implementation and trace how converter logic is currently integrated there. Review the surrounding connector flow before defining the Converter object boundary. Done means the converter logic is abstracted into the corresponding Converter object without changing the stated sendRecord behavior; no test file is named in the issue.

Written by the indexing model from the issue text.