Refactor SchedulingService: node_assignment task

What does this MR do and why?

This MR updates the node_assignment task to assign a batch of namespaces to multiple nodes by predicting the disk usage after assigning every new namespace.

#439042 (closed)

DB plans for ::Search::Zoekt::Node.online

SELECT "zoekt_nodes".* FROM "zoekt_nodes" WHERE "zoekt_nodes"."last_seen_at" >= '2024-02-01 13:19:41.161099' LIMIT 1000

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/25936/commands/81619

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.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #439042 (closed)