Allow nodes with from_env/from_zk references and replace=1 in configuration file

Why it matters

This feature enables more flexible and dynamic configuration management by allowing configuration values to be sourced from environment variables or ZooKeeper. It also supports overriding configuration nodes cleanly via the replace=1 attribute, improving configuration maintainability and adaptability in distributed or containerized environments.

How to use it

In the ClickHouse configuration XML file, define nodes using the from_env or from_zk attributes to refer to environment variables or ZooKeeper paths respectively. To override an existing configuration node, ensure the node is non-empty and set replace="1" on the element that should replace it.