allow setting multiple blinded paths and multiple hops per path in offers

1. Only have private channels (required because of https://github.com/ElementsProject/lightning/issues/7713) and no public channels.
2. Create new offer
3. decode the offer
4. Notice that there is a single blinded path and the `first_node_id` is only 1 hop away from my node

Would be useful to be able to include multiple blinded paths and also multiple hops. The current way is not very reliable or private.

Note, I have a single channel to another node with a lot of channels in my current tests. It's possible that you have the code configured to only have one blinded path per channel peer. I think that is a decent approach if you have multiple channel peers, but if you have a single channel peer, you should have multiple routes outside of that single channel peer instead.