Support updates to Marketplace-related webhooks and REST APIs

[gmlewis]

GitHub Developer API announcement:
https://developer.github.com/changes/2018-10-31-improved-experience-for-marketplace-pending-changes/

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!

[srmocher]

@gmlewis New contributor here. Interested in working on this, could you assign this issue to me?

[gmlewis]

Thank you, @srmocher!
I'm no longer able to send a repo contributor invite to you, but please consider yourself assigned. 😄

[dreamer-89-zz]

@gmlewis : I am also interested to work on this issue. It seems @srmocher working on this one. Can you please assign me other issues/bugs ?

[gmlewis]

Thanks for your interest, @dreamer-89!

My recommendation to you is to go through the list of open issues in this repo and choose one that interests you... then add a comment that you want to work on it. (I just browsed through and there are a bunch of outstanding ones.)

If someone is already listed on it, chances are high that they have forgotten about it and you can ask to take it over from them.

[dreamer-89-zz]

Thank you so much @gmlewis :) :)

[dreamer-89-zz]

@gmlewis : Please review
#1041

[gmlewis]

@dreamer-89 - sorry I wasn't clear before... you apparently misunderstood me. This issue is assigned to @srmocher and so you should not be submitting PRs against this issue. I started to review your PR and came across a number of issues (both with the PR and with the GitHub documentation), then came back here and realized that this issue is not yours (currently) to be working on.

As a result, I'm going to close your PR and let's give @srmocher a chance to submit a PR against this issue.

Thank you very much for your enthusiasm, but please go back through the list of other open issues and chose one of those to start contributing to. If it is already assigned (like this one... you have to read the comments to determine that, unfortunately, as I no longer have permissions in this repo to send contributor invites), please first ask if you can take over the issue, but only if they've had a fair amount of time (a week or two maybe) to work on it.

[gmlewis]

Note that to fully support this issue, there is a wealth of information located here:
https://developer.github.com/apps/marketplace/integrating-with-the-github-marketplace-api/github-marketplace-webhook-events/
that needs to be read and understood.

Specifically, any newly-created model structs should fully document the possible values of enumerated string fields (e.g. state).

[willnorris]

(invites sent to @srmocher and @dreamer-89. sorry about the delay)

[sionleroux]

If this issue is resolved by #1042 then we could close it. Or is there something that still needs doing here?

[gmlewis]

Thank you, @sinisterstuf - I believe you are correct that this can be closed.
Feel free to reopen if anyone disagrees.