Add flow run urls to notifications

[zangell44]

This PR adds flow_run_url as an available template variable for flow run notifications. We had this feature in v1 and it's a popular request for v2.

The default notification template now includes both the flow run url and state message.

Closes #6805 & https://github.com/PrefectHQ/prefect/issues/6604

Example

Now you can create a Slack webhook with a message like

Flow run bad-example now in state Completed. Read more detail here <link to flow run>.

Checklist

• [x] This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• [x] This pull request includes tests or only affects documentation.
• [x] This pull request includes a fix, feature, enhancement, docs, collections, or maintenance label categorizing the change.

[netlify[bot]]

✅ Deploy Preview for prefect-orion ready!

Name	Link
🔨 Latest commit	84a5340040e8feeec83114d74b9e836c004dd905
🔍 Latest deploy log	https://app.netlify.com/sites/prefect-orion/deploys/632c6c293f3bda0008bd899f
😎 Deploy Preview	https://deploy-preview-6798--prefect-orion.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[zhen0]

Could we add the flow run url to the default message too?

[zangell44]

Could we add the flow run url to the default message too?

Sure thing!

[zhen0]

I've tested this locally and looks good to me. Could we get a code review and get this merged before the next release?