"I am developing an LTI provider app, which has been going great, up until I tried to build out the Grade Passback feature. (Grade Passback Tools - Canvas LMS REST API Documentation). I am able to POST the necessary XML string/content to the provided lis_outcome_service_url but when I do, I receive an "Invalid authorization header" error. I assume that I am missing the required OAuth headers from my POST request, but I'm not even sure what I'm missing or how to implement the missing header(s). I am developing in PHP and using the Guzzle library to perform my POST request. Can someone please enlighten me on what I'm doing wrong or how I can go about fulfilling the required authorization for my POST request?