Custom blinded paths for invoices

[Lagrang3]

I noticed that I can create offers with custom blinded paths with the option dev_paths.
But I also noticed that the invoice derived from that offer does not contain the same blinded paths as the offer.
Instead I get the default (and useless) blinded path with a single hop, the destination in plain sight.

Run this tests to check

def test_offers_paths(node_factory):
    l1, l2, l3, l4 = node_factory.line_graph(4, wait_for_announce=True)
    path = [l2.info['id'],l3.info['id'],l4.info['id']]
    
    # l1 pays, outside blinded path
    offer = l4.rpc.offer(amount="21sat",description="test_offers_paths1",
        dev_paths=[path])['bolt12']
    offer_path = l1.rpc.decode(offer)['offer_paths'][0]
    assert len(offer_path['path'])==3
    assert offer_path['first_node_id']==l2.info['id']
    invoice = l1.rpc.fetchinvoice(offer)['invoice']
    invoice_path = l1.rpc.decode(invoice)['invoice_paths'][0]
    assert len(invoice_path['path'])==3
    assert invoice_path['first_node_id']==l2.info['id']

It fails at assert len(invoice_path['path'])==3 and assert invoice_path['first_node_id']==l2.info['id'].

It is ok that the invoice_paths could be different from the offer_paths, but if I specified that I wanted
to use those paths in the offer, since I have no control of the invoice generation, I would expect
those paths to be used in the invoice as well.

[21M4TW]

I have been working on related improvements of the code lately, and I come across this issue as well (while working on this PR). I think the same paths as the offer should be used. However currently it is not possible for the node generating the invoice (the payee) to know this path, as it does not have the path_privkey to unblind the offer paths anymore when the invoice request is received. I am working on a way to address this issue first, then it will be possible to generate the same path in the invoice as used in the offer.

[21M4TW]

In this branch I do recover the offer blinded paths' node IDs when an invoice request is received. However I have yet to produce blinded paths for the invoice using these, as the function found_best_peer from plugins/offers_invreq_hook.c will have to be modified substantially to support multiple hops. I will work on this when I have the chance.