Feat: add write to github implementation

The github API allows adding, updating or removing files ( https://docs.github.com/en/rest/repos/contents?apiVersion=2022-11-28#create-or-update-file-contents ).

The wrinkle is, that each action requires a commit message (as if you had edited in the github UI) and for update/delete, the sha of the previous commit (for avoiding races). 