Improvement: s3_scan_object API Lambda function invoke

[patheard]

Summary

Currently the s3_scan_object Lambda function makes and HTTPS request to trigger the scan of a new S3 object. It would be more efficient to have the s3_scan_object directly invoke the API Lambda function and allow us to use IAM policies rather than the API auth token to authenticate the request.

As an added benefit this would also allow us to prevent Staging instances of the s3_scan_object Lambda from making requests against the Prod API.