Ability to update a Flag activity that has already been sent to an inbox

[ThisIsMissEm]

Is your feature request related to a problem? Please describe.

When we send a Flag activity, we send something like the following to https://other.social/:

    {
      "@context": "https://www.w3.org/ns/activitystreams",
      "id": "https://social.example/reports/123",
      "type": "Flag",
      "content": "Harassing comment",
      "actor": "https://social.example/@jane_smith",
      "object": [
         "https://other.social/users/123",
         "https://other.social/users/123/status/456",
      ],
    }

This is fine, but what happens if we want to update the contents of that Flag? e.g., maybe we discovered more content that falls under the same report, or maybe we need to update the ID because we originally used IRIs that weren't dereferencable (mastodon does this) but now we want to make them dereferenceable.

This would require some way to update the Flag activity we sent https://other.social/, in the ActivityStreams issue tracker this was first mentioned in regards to the recipient of the Flag activity sending back to the flagger "hey, we've taken some action", which proposed something like this:

   {
      "@context": "https://www.w3.org/ns/activitystreams",
      "type": "Update",
      "object": {
        "type": "Flag",
        "id": "https://social.example/reports/123",
        "content": "Harassing comment",
        "actor": "https://social.example/@jane_smith",
        "object": [
           "https://other.social/users/123",
           "https://other.social/users/123/status/456",
        ],
      }
   }

Though I'm not sure if that is actually the "correct" way to do this, since an Activity isn't quite an Object?

Additional context

This is related to #7, however, is distinct as it's above the sender of the Flag activity sending an update of that Flag activity.