Improve handling of non-existing hosts in user allowed host list

Why it matters

This feature addresses issues when a user’s allowed host list contains non-existing hosts. It enhances ClickHouse's behavior by properly managing these cases, ensuring more reliable and predictable access control based on host validation.

How to use it

No additional configuration is required. Just specify hosts in the user allowed host list as usual, and ClickHouse will now handle non-existing hosts gracefully.