storage: origin_visit_add: Type and make origin_visit_add return an OriginVisit object

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:

• swh-loader-core!378 (closed): swh-loader-core (runtime)
• swh-loader-svn!195 (closed): swh-loader-svn (runtime)
• swh-web!1000 (closed): swh-web (tests)
• swh-indexer!448 (closed): swh-indexer (tests)

Test Plan

tox

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Migrated from D2820 (view on Phabricator)