Add support for cluster_for_parallel_replicas with custom key in MergeTree tables

Why it matters

This feature addresses the limitation of using parallel replicas only with default keys by allowing the use of custom keys for parallel replicas. It improves query parallelism and performance on MergeTree tables with custom key configurations.

How to use it

To enable this feature, specify the cluster_for_parallel_replicas parameter when defining parallel replicas with a custom key in your MergeTree table configuration or query settings.