Add an option to send autograding output to the Feedback PR as a comment

[markpatterson27]

Currently, the output from autograding is hard to find for any learner new to GitHub. This makes it hard for learners to get feedback on which parts of their assignment they haven't completed yet. A better option would be for autograding to post a grading summary on the Feedback PR.

Feature request either autograding doing this directly, or provide a feedback step output that can be passed to other steps in a workflow.

I have a proof-of-concept setup here: https://github.com/markpatterson27/PoC-Autograding-Feedback/