CassandraSinkCluster: pull routing logic into separate function

[rukai]

CassandraSinkCluster has a really large send_message method, it should be broken down into multiple smaller functions.
To start with lets pull out the request routing logic into a route_requests method, this matches a function of the same name and purpose in KafkaSinkCluster.

The send_message function is also really poorly named but that should be addressed by moving the contents into Transform::transform which should occur in another PR to keep scope down.

The "Files changed" diff is very confusing and doesnt properly show the moving of the routing logic.
If you look at the individual commits you can see that the routing logic is removed and then readded as a separate function.
I recommend pasting the large chunk of logic that is removed and readded into a diff tool to verify the logic remains identical.

[codspeed-hq]

CodSpeed Performance Report

Merging #1631 will not alter performance

_{Comparing rukai:cassandra_sink_cluster_refactor_route_requests (31aa0ee) with main (618638a)}

Summary

✅ 37 untouched benchmarks