Add config for paths to a static invoice server

As part of being an async recipient, we need to support interactively building
an offer and static invoice with an always-online node that will serve static
invoices on our behalf.

Add a config field containing blinded message paths that async recipients can
use to request blinded paths that will be included in their offer. Payers will
forward invoice requests over the paths returned by the server, and receive a
static invoice in response if the recipient is offline.

Author: valentinewallace

lightning/src/util/config.rs

@@ -10,8 +10,10 @@
 //! Various user-configurable channel limits and settings which ChannelManager
 //! applies for you.
 
+use crate::blinded_path::message::BlindedMessagePath;
 use crate::ln::channel::MAX_FUNDING_SATOSHIS_NO_WUMBO;
 use crate::ln::channelmanager::{BREAKDOWN_TIMEOUT, MAX_LOCAL_BREAKDOWN_TIMEOUT};
+use crate::prelude::*;
 
 #[cfg(fuzzing)]
 use crate::util::ser::Readable;
@@ -878,6 +880,11 @@ pub struct UserConfig {
 	/// [`ChannelManager::send_payment_for_bolt12_invoice`]: crate::ln::channelmanager::ChannelManager::send_payment_for_bolt12_invoice
 	/// [`ChannelManager::abandon_payment`]: crate::ln::channelmanager::ChannelManager::abandon_payment
 	pub manually_handle_bolt12_invoices: bool,
+	/// [`BlindedMessagePath`]s to reach an always-online node that will serve [`StaticInvoice`]s on
+	/// our behalf.
+	///
+	/// [`StaticInvoice`]: crate::offers::static_invoice::StaticInvoice
+	pub paths_to_static_invoice_server: Vec<BlindedMessagePath>,
 }
 
 impl Default for UserConfig {
@@ -891,6 +898,7 @@ impl Default for UserConfig {
 			manually_accept_inbound_channels: false,
 			accept_intercept_htlcs: false,
 			manually_handle_bolt12_invoices: false,
+			paths_to_static_invoice_server: Vec::new(),
 		}
 	}
 }
@@ -910,6 +918,7 @@ impl Readable for UserConfig {
 			manually_accept_inbound_channels: Readable::read(reader)?,
 			accept_intercept_htlcs: Readable::read(reader)?,
 			manually_handle_bolt12_invoices: Readable::read(reader)?,
+			paths_to_static_invoice_server: Vec::new(),
 		})
 	}
 }