origin_visit_add: Adapt endpoint signature to return OriginVisit

Prior to this commit, there was:

• no signature in the method
• discrepancy between checks on the different backend

origin_visit_add endpoint is now typed

def origin_visit_add(
    self, origin_url: str,
    date: Union[str, datetime.datetime], type: str) -> OriginVisit:

This also:

• renames appropriately the origin_url parameter (removing 1 FIXME)
• align backend implementations' check which were different