Feat: Add 'Date Last Assigned' to Reviewer Spotlight

[adeyinkaoresanya]

To improve the maintainer's ability to distribute badging application review workload effectively from a single source of truth, we need to track when a reviewer(contributor) was most recently assigned to an issue. This will also help reflect contributors' actual assignments rather than just general activity.

Key Requirements:

1. The system must distinguish between historical assignments and current ones.
2. If a reviewer was assigned and then unassigned (e.g., to swap in someone else), the "Last Date Assigned" date should not reflect that specific unassigned session. It should only store the date of the currently active assignment.

Tests
To ensure the logic is robust (especially for your sandbox environment), test these four scenarios:
Simple Assignment: User is assigned once. Logic returns that date.
Re-assignment: User A is assigned, unassigned, then assigned again. Logic returns the second (latest) assignment date.
Un-Assignment: User A was assigned and then unassigned. Logic returns None (because they aren't currently assigned).
Multiple Assignees: User A and User B are both assigned. Logic should correctly distinguish and return the unique date for whichever user you are querying.

Update: After reviewing PR #495, we realized that the original description focused only on active assignments, interpreted as open issues.

Clarified goal: The “Date Last Assigned” should reflect the last time each reviewer was assigned to an issue, regardless of whether the issue is currently open or closed, while still ignoring sessions where they were unassigned.

This helps the badging lead understand how recently a reviewer was assigned before selecting them for the next review.

[rwejoye]

Hello @adeyinkaoresanya, I would like to work on this. Could you please assign it to me?

[adeyinkaoresanya]

Hello @adeyinkaoresanya, I would like to work on this. Could you please assign it to me?

Sure thing, @rwejoye!