Add system.user_processes table and SHOW USER PROCESSES query for user-level memory info and ProfileEvents

Why it matters

This feature provides a higher-level overview of resource consumption by user, allowing database administrators to monitor and diagnose memory usage and performance events per user. It simplifies tracking the impact of individual users on system resources.

How to use it

Users can query the new system.user_processes table directly using a SELECT statement, or use the shorthand command SHOW USER PROCESSES to retrieve aggregated memory and ProfileEvents metrics for each user.