Add CLUSTER Grant and Backward Compatibility Directive

Why it matters

This feature adds finer-grained access control for cluster-wide queries in ClickHouse by allowing administrators to require a distinct CLUSTER grant. It addresses the need for improved security and permission management when running queries across clusters. The backward compatibility is maintained via the access_control_improvements.on_cluster_queries_require_cluster_grant configuration, which defaults to false to preserve existing behavior.

How to use it

To enable this feature, set the configuration parameter access_control_improvements.on_cluster_queries_require_cluster_grant to true in the server configuration. After enabling, assign the new CLUSTER grant to users who need permission to execute cluster queries.