Expose `created_at` field in Tags API

What does this MR do and why?

Contributes to #451011 (closed)

Problem

Tags API does not provide information about tag's creation timestamp for annotated tags.

Note Lightweight tags don't have their own timestamp.

Solution

• Add field created_at to Tags API response.

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

created_at is present in the response

Before	After

How to set up and validate locally

1. Visit http://gdk.test:3000/api/v4/projects/2/repository/tags
2. You should see a new field created_at in the response