Added part paths information to system.merges

Why it matters

This feature provides users with visibility into the file system paths of data parts involved in merges. It helps in diagnostics and monitoring by allowing users to track exactly which parts are being merged and where they are located on disk.

How to use it

Users can query the system.merges system table to retrieve additional columns containing part path information as part of the merge process. No special configuration is required; the information is available by default after the update.