(BSR)[API] fix: Fix non-thread safe usage of boto3 client in outscale connector

[dbaty]

boto3.client() is not thread-safe. We sometimes get the following
error:

KeyError: 'default_config_resolver'
  File "pcapi/tasks/ubble_tasks.py", line 23, in store_id_pictures_task
    ubble_subscription_api.archive_ubble_user_id_pictures(payload.identification_id)
  [...]
  File "pcapi/connectors/beneficiaries/outscale.py", line 13, in upload_file
    client = boto3.client(
  File "__init__.py", line 92, in client
    return _get_default_session().client(*args, **kwargs)
  [...]
  File "botocore/session.py", line 1009, in get_component
    del self._deferred[name]

To work around that, we now use a dedicated session. And we keep one
for each thread to lessen the overhead of the session initialization.

References:

• discussion of the issue and possible workarounds: Exceptions coming from boto3/botocore when running boto3.client('sts') too many times simultaneously boto/boto3#1592
• similar fix in Sentry client code: getsentry/sentry@f58ca5e

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L'erreur sur Sentry : https://sentry.passculture.team/organizations/sentry/issues/408606/?project=5&query=default_config_resolver&statsPeriod=90d