Go-github: Add missing fields to MarketplacePlan and MarketplacePurchase structs

[SudoVim]

According to the GitHub documentation: https://docs.github.com/en/free-pro-team@latest/rest/reference/apps#list-accounts-for-a-plan the "number" field is missing from MarketplacePlan, and the "updated_at" field is missing from MarketplacePurchase. I've already made these changes in my fork, so I'd like to submit a PR for review. However, it might take me a little while to get approval for this license agreement from my employer.

[gmlewis]

Thank you, @SudoVim .

I'll open this up to any volunteer contributor so that you don't have to worry about making the PR yourself.

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!

[chughpiyush]

@gmlewis I can take up this one :)