Add round_robin load_balancing mode

Why it matters

This feature solves the problem of uneven query distribution among replicas by enabling a round-robin strategy, which helps balance the load more fairly and efficiently across all available replicas. It improves resource utilization and reduces bottlenecks caused by uneven replica usage.

How to use it

To use the round_robin load_balancing mode, configure your replica cluster or setting to specify round_robin as the load_balancing strategy. This will make ClickHouse distribute queries to replicas in a cyclic fashion.