Go-github: Support "package" webhook event

[patrickmarabeas]

Add support for the package webhook event.

Docs: https://developer.github.com/webhooks/event-payloads/#package

[gmlewis]

Thank you, @patrickmarabeas!

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.

Thank you!

[ravindranrahul]

Hi @gmlewis ,

If this is still relevant, I would like to give this a try.

[gmlewis]

Thank you, @ravindranrahul! It is yours.

[jeet-parekh]

@ravindranrahul, any updates on this?