scheduler: Type and unify get_scheduler factory with other factories

Impacts:

$SWH_ENVIRONMENT_HOME/swh-deposit/swh/deposit/config.py:        self.scheduler: SchedulerInterface = get_scheduler(**self.config["scheduler"])
$SWH_ENVIRONMENT_HOME/swh-deposit/swh/deposit/tests/conftest.py:        scheduler = get_scheduler(**cfg["scheduler"])
$SWH_ENVIRONMENT_HOME/swh-lister/swh/lister/core/lister_base.py:        self.scheduler = get_scheduler(**self.config["scheduler"])
$SWH_ENVIRONMENT_HOME/swh-scheduler/swh/scheduler/api/server.py:        scheduler = get_scheduler(**app.config["scheduler"])
$SWH_ENVIRONMENT_HOME/swh-scheduler/swh/scheduler/__init__.py:def get_scheduler(cls: str, **kwargs) -> SchedulerInterface:
... # snip scheduler impacts dealt with here
$SWH_ENVIRONMENT_HOME/swh-vault/swh/vault/__init__.py:        args["scheduler"] = get_scheduler(**args["scheduler"])
$SWH_ENVIRONMENT_HOME/swh-web/swh/web/config.py:        swhweb_config["scheduler"] = get_scheduler(**swhweb_config["scheduler"])

Note: Those should still work ok, only a deprecation warning should show up

Related to swh-core#1410

Depends on !157 (closed)

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Migrated from D4284 (view on Phabricator)