Add Support for SHOW MERGES Query

Why it matters

The SHOW MERGES query allows users to monitor the merge operations happening within ClickHouse tables. This feature helps in understanding background activity related to data merging, improving observability and management of storage optimization processes.

How to use it

Users can execute the SHOW MERGES SQL command to retrieve information about current and recent merges. For example:

SHOW MERGES;

This will display details of merges such as table names, partitions, and merge progress.