bolt12: adding post-processing hook for onion messages

[hydra-yse]

Current Issue:
Currently, there is an issue regarding incoming payments which require opening a channel, as the invoices generated from fetch_invoice do not take into consideration opening channel fees, meaning that once the payer attempts paying said invoice, the recipient's node will reject it with an "invalid amount" error.

Proposed Solution:
A solution to this would be to specify a post-processing hook to the fetch_invoice request which allows us to modify the outgoing payload.
Ideally, the hook should allow us to parse the generated invoice and create a new one with the updated amount, so that it can be forwarded back to the payer.

[JssDWt]

As discussed with @cdecker, probably the ideal interface would be when the hook:

• is called with the initial bolt12 invoice created by cln
• allows modifying that invoice
• has a flag to signal you don't want the invoice to be persisted on the node, in case you do that manually inside the hook, but return a different invoice for the requester.