Add `Invoice::void` method

[ferrohd]

Is your feature request related to a problem? Please describe.

The Invoice object has some useful ext methods such as:

• Invoice::upcoming
• Invoice:finalize
• Invoice::pay
• Invoice::delete

Invoices generated from a Subscription cannot be deleteed, instead must be voided. My feature request is to add Invoice::void.

Describe the solution you'd like

Add in the Invoice::void method, used to void invoices.

Describe alternatives you've considered

Invoice::void, like Invoice::delete feels like should already exists in the OpenAPI spec instead of adding it to invoice_ext

Additional context

I can try to work on the PR.