Add merge_reason column to system.part_log table

Why it matters

This feature provides additional insight into the reasons why parts are merged within ClickHouse. It helps users and DBAs better understand merge operations by explicitly logging the cause of each merge event, improving observability and debugging of merges.

How to use it

Users can query the system.part_log table and include the merge_reason column in their SELECT statements to analyze merge events and their causes.