Custom blinded paths for invoices

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
```python
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.