Async invoice creation

[ekzyis]

Describe the problem you're trying to solve

Receiver wallets can take pretty long to create invoices. Since our mutation waits for wallets to create an invoice, our load balancer might timeout before this happens.

Describe the solution you'd like

Instead of increasing timeouts for wallets and load balancer, we could return an id that the frontend can use to poll for the invoice.

This might not be simply the invoice id since we only store invoices in the database when we actually have something to store.

Describe alternatives you've considered

No response

Additional context

Doing #1717 first might be useful.

#1492 might also be related since this ticket is also about sending invoices to the client. Ideally, the code for #1492 can also be used for async invoice creations.