Add event timestamp to jobs webhooks
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Broken off of https://gitlab.com/gitlab-org/gitlab/-/issues/264393
### Release notes
Visualizations of jobs webhook data now have an event transition timestamp event to anchor chronologically.
<!-- What is the problem and solution you're proposing? This content sets the overall vision for the feature and serves as the release notes that will populate in various places, including the [release post blog](https://about.gitlab.com/releases/categories/releases/) and [Gitlab project releases](https://gitlab.com/gitlab-org/gitlab/-/releases). " -->
### Problem to solve
Some webhooks (jobs and deployments for example) from gitlab.com webhooks do not contain timestamp information in their json payload. This is usually found in object_attributes -> created_at in other webhooks.
Timestamps are helpful to retain real-time data stream of events for downstream actions. Part of these actions may want to understand when the event actually occurred and the best guess is when the webhook was received. This creates data integrity challenges and doesn't allow the downstream systems to draw visualizations based on actual event times.
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### Intended users
* [Cameron (Compliance Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#cameron-compliance-manager)
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#delaney-development-team-lead)
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
* [Devon (DevOps Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
* [Priyanka (Platform Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#priyanka-platform-engineer)
* [Dana (Data Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#dana-data-analyst)
### User experience goal
webhooks received from gitlab.com (or self-managed gitlab) show a timestamp when the event occured.
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### Proposal
Add a `timestamp` or `event_timestamp` to each webhook payload which represents when the event occured.
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issue