This aligns the last writing endpoints to use BaseModel (Origin, OriginVisit) objects as input and OriginVisit as output.
After adding the type effectively used, here is the comparison:
```
before def origin_visit_add(self, origin: Dict, date: Union[str, datetime.datetime], type: str) -> Optional[Dict[str, Union[str, int]]]:
after def origin_visit_add(self, origin: Origin, date: Union[str, datetime.datetime], type: str) -> OriginVisit:
```
The results is both 'origin' and 'visit', but only visit interests us here...
So `origin_visit_add` will be further simplified in the next diff D2821
Impacts:
- swh-loader-core (runtime)
- swh-loader-svn (runtime)
- swh-journal (tests)
- swh-web (tests)
- swh-indexer (tests)
Depends on D2813