Seems like we can do this with [`FieldTransform.append_missing_elements`](https://firebase.google.com/docs/firestore/reference/rpc/google.firestore.v1). JIRA: [FD-162](https://filify.atlassian.net/browse/FD-162) [FD-162]: https://filify.atlassian.net/browse/FD-162?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
Seems like we can do this with
FieldTransform.append_missing_elements.JIRA: FD-162