Add nonce to OffersContext::OutboundPayment

Add nonce to OffersContext::OutboundPayment

To authenticate that a Bolt12Invoice is for a valid InvoiceRequest or
Refund, include the nonce from the payer_metadata in the InvoiceRequest
reply path or Refund::paths, respectively. This can be used to prevent
de-anonymization attacks where an attacker sends invoices using
self-constructed paths to nodes near the blinded paths' introduction
nodes.