Look and support the new bolt12 flow

[vincenzopalazzo]

We need a way to implement the new bolt12 flow like lightningdevkit/rust-lightning#3833

This introduces a way to mess with the bolt12 flow and allow not to use a hack around it like forking ldk https://github.com/vincenzopalazzo/lampo.rs/tree/lampo-v0.1.4

[vincenzopalazzo]

This is blocked by understanding how the bolt12 flow works and seeing how and when it will be supported fully in LDK, we can help to speed up stuff!

cc @jkczyz