Add created_at to job webhooks

Problem to solve

Add "created_at" field to job webhook, to improve consistency with the API response for jobs.

Intended users

User experience goal

Improved data consistency across job API and job webhooks.

Proposal

Add the "created_at" time field to the job webhook return - it already exists in the job API responses, so presumably just put it into the job webhook using same timestamp format and value.

Further details

Right now, if we want to know 100% when a job was created based on a webhook we receive, we need to call the gitlab API to get this data, which essentially negates a lot of the benefits we get with webhooks.

Permissions and Security

No changes to security anticipated, just an update to webhook data.

Documentation

Availability & Testing

What does success look like, and how can we measure that?

created_at field exists in the webhook data for jobs, and matches the value that is returned from the job API for the specific jobs.