KafkaSinkCluster - Split OffsetFetch requests

[rukai]

refer to comment:

shotover-proxy/shotover/src/transforms/kafka/sink_cluster/mod.rs

Lines 733 to 739 in 985668f

	// This is possibly dangerous.
	// The client could construct a message which is valid for a specific shotover node, but not for any single kafka broker.
	// We may need to add some logic to split the request into multiple messages going to different destinations,
	// and then reconstruct the response back into a single response
	//
	// For now just pick the first group as that is sufficient for the simple cases.
	offset_fetch.groups.first().unwrap().group_id.clone()

similar to #1666