vault: add monitoring of bundle downloads
This adds a download step to the vault end to end check once the API claims that the cooking was successful.
The download step checks:
- that the fetch_url is present
- that the status code is successful
- that the response is not empty
Related to swh/infra/sysadm-environment#3983 (closed)
Test Plan
new test scenarios added for the new failure modes
Migrated from D7264 (view on Phabricator)