KafkaSinkCluster: improve message naming

[rukai]

Where possible we should avoid the variable name message instead use either request or response. This makes the code easier to read since it gives more context to the type of message being stored.

[codspeed-hq]

CodSpeed Performance Report

Merging #1787 will not alter performance

_{Comparing rukai:prefer_request_naming (8c3faba) with main (487ffec)}

Summary

✅ 38 untouched benchmarks