Add <storage_policy> config parameter for system logs

Why it matters

This feature allows users to specify a storage policy for system logs, enabling better control over their storage location, retention, and management. It helps improve operational flexibility and resource management by aligning system log storage with user-defined policies.

How to use it

To use this feature, add the <storage_policy> parameter to the system logs configuration section in the ClickHouse server config file, specifying the desired storage policy name.