Add merge_algorithm to system.merges table for better merging inspections

Why it matters

This feature helps users better inspect and analyze merge processes by showing which merge algorithm is being applied. It improves transparency and troubleshooting capabilities related to merges in ClickHouse.

How to use it

Users can query the system.merges table and view the new merge_algorithm column to see the merge algorithm used for each merge operation. For example:

SELECT database, table, merge_algorithm, description FROM system.merges;