Add setting always_fetch_merged_part to restrict replica merging and prefer downloads from other replicas

Why it matters

This feature addresses situations where it is preferable for replicas to avoid merging parts by themselves and instead always fetch the merged parts directly from other replicas. It helps optimize replica synchronization and potentially reduces local merge overhead, ensuring data consistency and efficient replication.

How to use it

Enable the feature by setting always_fetch_merged_part to true in the replica's configuration or session settings. When enabled, the replica will restrict itself from merging parts locally and will prefer downloading merged parts from other replicas.