Add a `created_at` field

We have the last_modified field, but sometimes we would love to have the created_at ones too.

Here is how a parse record look like:

```
{
"updatedAt": "2011-08-21T18:02:52.248Z",
"createdAt": "2011-08-20T02:06:57.931Z",
"objectId": "Ed1nuqPvcm",
}
```

Start by tracing how parse records expose the existing last_modified field and where record metadata is serialized or returned. Add a created_at value alongside the existing timestamp, using the format shown in the issue, and verify that parse records include both fields without changing updatedAt behavior.

Written by the indexing model from the issue text.