posthog-js supports person.set_once but our backend doesn't. Make sure we can pass through `$set_once` on an `$identify` call, and only set the properties within if the properties aren't already set on the user.
ee/clickhouse/process_event.py
event_uuid: UUID,
    person_uuid: UUID,
    ip: str,
    site_url: str,
    team_id: int,
    event: str,
    distinct_id: str,
    properties: Dict,
    timestamp: datetime.datetime,
) -> None:
    elements = properties.get("$elements")
    elements_list = []
    if elements:
        del properties["$elements"]
        elements_list = [
            Element(
                text=el["$el_text"][0:400] if el.get("$el_text") else None,
                tag_name=el["tag_name"],
                href=el["attr__href"][0:2048] if el.get("attr__href") else None,
                attr_class=el["attr__class"].split(" ") if el.get("attr__class") else None,
                attr_id=el.get("attr__id"),
                nth_child=el.get("nth_child"),