Add throttling to bulk assignments

What does this MR do and why?

Add throttling to bulk assignments

This MR adds throttling to the bulk user assignments to have less impact on database if run for large number of users.

The script is primarily used by Self-Managed instance admin via rake task. For once, this will be also used by GitLab SRE to assign Gitlb.org group members seats: #441776. This will allow all gitlab members have access to Code Suggestions/Duo Chat without interruptions when we switch to paid model on 15th Feb.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Green CI tests.

Related to #441776