Add Merge_max_block_size Setting to Mergetreesettings #7412 (artem Zuikov)

Why it matters

This feature allows users to limit the block size used during background merges in MergeTree family storage engines, helping to manage memory consumption and potentially improve merge performance and stability.

How to use it

Set the merge_max_block_size setting in the server configuration or as a session-level parameter to specify the maximum block size for merges. For example:

SET merge_max_block_size = 1048576;

Adjust this value based on workload characteristics and available system resources.