bolt12: adding support for `fetch_invoice` outbound hook

[hydra-yse]

The goal is to build atop the preexisting hooks and provide a method for Greenlight to intercept incoming and outgoing onion messages related to invoice generation and react accordingly.

Further details about the Core Lightning implementation of the hook can be found at ElementsProject/lightning#6933

[legioz]

+1 for BOLT12 support! With CLN already production-ready for offers, bringing BOLT12 to Greenlight would unlock powerful UX improvements for recurring payments, donations, and privacy. It’s a key step to keep Greenlight aligned with the Lightning ecosystem’s evolution.

[cdecker]

Is the hook implemented in CLN? If not, that has to happen first.