[feature] Snapshot images in Slack notifications

**Version and OS**
v0.49.4 on Docker

**Is your feature request related to a problem? Please describe.**
I currently have `ChangeDetection` integrated with `Browserless` for taking snapshots of websites to detect changes. I also have a notification integration with Slack and have toggled the `Attach screenshot to notification` option.

Unfortunately, when notifications come into Slack, they do not contain a screenshot. The Slack API does support attachments in messages posted via their API however.

**Describe the solution you'd like**
When a change is detected and Slack is notified, a snapshot is shared in the Slack message body.

**Describe the use-case and give concrete real-world examples**
Notification body:
```
There's a new update available!

{{watch_url}}
```

If any additional details or examples are needed, please let me know!

Start by tracing the existing Slack notification integration and the “Attach screenshot to notification” option, then inspect how Browserless snapshots are made available when a change is detected. Confirm how Slack message attachments are posted through its API; done means a detected change delivers the snapshot in the Slack message.

Written by the indexing model from the issue text.