Validate a storage connection

Start a storage_validation job that checks Nace can write, read and delete in the destination, from both its API and its workers. Read it with GET /v1/storage-connections/{id}/validation-job; once it succeeds the connection is ready. A new IAM role or Azure role assignment can take a few minutes to apply, so a fresh one may fail at first. Free. Only where customer storage is on for the deployment; where it is off, every storage connection route answers 404 storage_connection_unavailable once the API key is accepted, before checking the request or its body.