Adds the boilerplate needed for PeerManager and OnionMessenger to work
together, with some corresponding docs and misc updates mostly due to the
PeerManager public API changing.
Also update the fuzz ChaCha20Poly1305 to not mark as finished after a single
encrypt_in_place. This is because more bytes may still need to be encrypted,
causing us to panic at the assertion that finished == false when we go to
encrypt more.
Also fix unused_mut warning in messenger + add log on OM forward for testing
This required adapting `onion_utils::decode_next_hop` to work for both payments
and onion messages.
Currently we just print out the path_id of any onion messages we receive. In
the future, these received onion messages will be redirected to their
respective handlers: i.e. an invoice_request will go to an InvoiceHandler,
custom onion messages will go to a custom handler, etc.
This adds several utilities in service of then adding
OnionMessenger::send_onion_message, which can send to either an unblinded
pubkey or a blinded route. Sending custom TLVs and sending an onion message
containing a reply path are not yet supported.
We also need to split the construct_keys_callback macro into two macros to
avoid an unused assignment warning.
