Add FROM_UNIXTIME function for MySQL compatibility

Why it matters

This feature provides users migrating from MySQL or integrating with MySQL-like environments the ability to use the familiar FROM_UNIXTIME function in ClickHouse, simplifying query compatibility and reducing migration effort.

How to use it

Users can use the function by calling FROM_UNIXTIME(timestamp), where timestamp is a Unix timestamp value (seconds since epoch). This will return the corresponding datetime value.