Add support for GitHub App Manifest flow on Organization, not User about go-github

[wenked]

Hi , im a new go developer and i would like to have a opportunity to work on this issue.

[gmlewis]

Thank you, @fproulx-boostsecurity. Would you like to create a PR or shall I open this up to other contributors to this repo?

[fproulx-boostsecurity]

@gmlewis better open the floor to others. I have other projects that keep me busy now. I noticed that OpenShift Pipelines as code was using this feature and it fell short because of that, but I hacked my way around it and overrode the URL using DevTools for today.

Would be nice for that to be addressed though so consumer of the lib would have better experience.

[gmlewis]

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue and the issue can be assigned to you so that others don't attempt to duplicate the work.

Please check out our CONTRIBUTING.md guide to get started. (In particular, please remember to go generate ./... and don't use force-push to your PRs.)

Thank you!

[gmlewis]

Thank you, @wenked - it is yours.

[fproulx-boostsecurity]

Awesome @wenked , looking forward to see the PR 👍