feat(invoice): stripe app invoice sync

[hekike]

Synchronize OpenMeter invoices with Stripe Invoicing through apps.

How it works

The invoice state machine calls the interfaces of compatible apps like Stripe App to Upsert and Finalize the invoice.
Driven by the state machine, the Stripe App creates, updates, and finalizes a Stripe Invoice object via the Stripe SDK.

If the invoice doesn't exist yet (determined by checking its external invoicing app ID), it is created; otherwise, it is updated in Stripe. Every OpenMeter invoice line and discount is translated to flat InvoiceLineItem(s) in Stripe. Discounts are represented as negative amounts. The update diffs the line items between Stripe and OpenMeter and decides which lines need to be updated, added, or removed in Stripe.

We prioritize synchronizing lines with quantity and per-unit price for greater visibility when they fit Stripe amount cents. When any Stripe line doesn't fit cents without floating points, we synchronize all lines with the total amount (the product of quantity and unit price).

Currently, automatic tax collection is enabled.

Todos:

• Add discount external id (feat(billing): add external id to invoice line discount #1989)
• Use external IDs to map Stripe line item to OM Line item instead of metadata
• Minimum spend Total amount is zero