Feature: More granular pay offer endpoints

[orbitalturtle]

Feature Description

In addition to pay-offer, which completes the full offer flow when sending a BOLT12 payment, we should have two other endpoints that offer a little more granular control when paying an offer.

• Get_invoice would, when provided an offer, get an invoice from the offer and return it.
• Pay_invoice would pay the invoice it's provided with.

This will require refactoring the pay_offer method on the offer handler. I'm imagining creating two methods of offer handler (get_invoice and pay_invoice) that mirror the above endpoints. Then we'd call these two methods in pay_offer, instead of clumping everything together in pay_offer.

[mauricepoirrier]

I can take this one! Is a bit bigger than the others, so i'll do a draft to collect early feedback

[mrfelton]

I can help you with testing and feedback on this one @mauricepoirrier

[mauricepoirrier]

@mrfelton cool ty! I'll ping you 🙌🏼

[orbitalturtle]

Closed by #131.