Add support for attachments with posts (notes)

Seems like it might be as easy as:

```
  "attachment": [
    {
      "type": "Image",
      "mediaType": "image/png",
      "url": "https://example.com/files/cats.png",
      "focalPoint": [
        -0.55,
        0.43
      ]
    }
  ]
```

https://docs.joinmastodon.org/spec/activitypub/#focalPoint