Multi-hop blinded paths in Offers

[jkczyz]

Currently, ChannelManager will create blinded paths with ourselves as the introduction node (i.e., one-hop blinded paths) for:

• Offer::paths and Refund::paths
• Onion message reply paths for InvoiceRequest and Bolt12Invoice
• Bolt12Invoice::payment_paths

To support multi-hop blinded paths, we could either

• trivially use "two" hops using a peer as the other hop, or
• use more than two by looking at the NetworkGraph