Support marketplace purchase event

[AlistairB]

👋 There are webhook events around github marketplace events (ie. a user installs a github app from the marketplace) that I would like to add support for.

See https://docs.github.com/en/developers/github-marketplace/using-the-github-marketplace-api-in-your-app/webhook-events-for-the-github-marketplace-api#github-marketplace-purchase-webhook-payload