Add setting to enforce read-through cache for merges

Why it matters

This feature allows users to enable read-through caching for merge operations, improving cache efficiency and potentially enhancing query performance by ensuring that merges utilize cached data effectively.

How to use it

To enable this feature, set the setting force_read_through_cache_for_merges to 1 in your ClickHouse configuration or per session. This will force merges to read data through the cache.