Added System Tables and SHOW Commands for Users, Roles, and Settings in ClickHouse

Why it matters

This feature enhances the administrative capabilities by providing direct access to metadata about users, roles, grants, settings profiles, quotas, and row policies through system tables. It also introduces new SHOW commands to simplify querying and managing security and configuration objects, improving ease of use and system transparency.

How to use it

Users can query the new system tables (e.g., system.users, system.roles, system.grants, system.settings_profiles, system.quotas, system.row_policies) to inspect current settings and access controls. Additionally, the new commands SHOW USER, SHOW ROLES, SHOW CURRENT ROLES, SHOW ENABLED ROLES, and SHOW SETTINGS PROFILES can be used to list and manage these objects directly from SQL queries.