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
Impacts:
- D2828: swh-loader-core (runtime)
- D2829: swh-loader-svn (runtime)
- swh-web (tests)
- swh-indexer (tests)