Issue advance invoice

Issues an invoice in advance for a subscription's next renewal date. See our docs for more information on advance invoices, including eligibility for generating one; for the most part, they function like any other invoice, except they are issued early and have special behavior upon being voided. A subscription may only have one advance invoice per billing period. Attempting to issue an advance invoice when one already exists will return an error. That said, regeneration of the invoice may be forced with the params force: true, which will void an advance invoice if one exists and generate a new one. If no advance invoice exists, a new one will be generated. We recommend using either the create or preview endpoints for proforma invoices to preview this advance invoice before using this endpoint to generate it.