Add NOT NULL for sharding key on issue_assignment_events

What does this MR do and why?

Adds NOT NULL constraint on sharding key column project_id on table: issue_assignment_events.

This merge request is the last step towards transforming the desired sharding key into a sharding key.

This involves the following changes:

• Adding a not null constraint for the sharding key column (namespace_id).
• Setting namespace_id as sharding_key of issue_assignment_events.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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