Feature: More granular pay offer

[a-mpch]

Should close #124

In this Pr:

• We separate in two methods and also endpoints to get_invoices and to pay_invoices
• We add an integration test (really hacky) to test getting an invoice and then paying it

Some details

1. I serialized the invoice using Readable from LDK newest versions and then hex the bytes as decribed in this comment
2. Another decision was to not use the interface from lnd and use the offer itself. This might be changed. Other implementations use the invoices as the parameter.

I didn't test as much I wanted but I'd like feedback first

[a-mpch]

@mrfelton might wanna check this 👀

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 0.00%. Comparing base (23fe019) to head (24fe6b4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #129 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 1 1 Lines 96 96 ====================================== Misses 96 96 ```

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[mrfelton]

Awesome. Any chance you could create a version that is rebased on top of https://github.com/lndk-org/lndk/pull/128? it would make testing a lot easier.

[a-mpch]

Awesome. Any chance you could create a version that is rebased on top of #128? it would make testing a lot easier.

I'll try ;)

[a-mpch]

closing, moved to #131