Hi,

Thanks for all the work on this library!

According to the docs, the MarketplacePurchase struct should have an Account field, but that field is currently missing.

I've checked the data sent by GitHub in the webhook, and we could either use the existing MarketplacePlanAccount type for the missing Account field, or create a new type something like this:

// MarketplacePurchaseAccount represents a GitHub Account (user or organization) for a specific purchase.
type MarketplacePurchaseAccount struct {
    Type                     *string    `json:"type,omitempty"`
    ID                       *int64     `json:"id,omitempty"`
    Login                    *string    `json:"login,omitempty"`
    OrganizationBillingEmail *string    `json:"organization_billing_email,omitempty"`
}

The benefit of adding the new MarketplacePurchaseAccount type is that we would avoid making the definition of MarketplacePurchase recursive. (The existing MarketplacePlanAccount type has a MarketplacePurchase field).

Please let me know if you'd like me to open a PR.