Add API to retrieve cached async receive offer

Over the past several commits we've implemented interactively building an async
receive offer with a static invoice server that will service invoice requests
on our behalf as an async recipient.

Here we add an API to retrieve this resulting offer so we can receive payments
when we're offline.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -10317,9 +10317,23 @@ where
 	#[cfg(c_bindings)]
 	create_refund_builder!(self, RefundMaybeWithDerivedMetadataBuilder);
 
+	/// Retrieve our cached [`Offer`] for receiving async payments as an often-offline recipient. Will
+	/// only be set if [`UserConfig::paths_to_static_invoice_server`] is set and we succeeded in
+	/// interactively building a [`StaticInvoice`] with the static invoice server, erroring otherwise.
+	#[cfg(async_payments)]
+	pub fn get_cached_async_receive_offer(&self) -> Result<Offer, ()> {
+		let mut offer_cache = self.async_receive_offer_cache.lock().unwrap();
+		offer_cache.check_expire_offer(self.duration_since_epoch());
+
+		offer_cache.offer.clone().ok_or(())
+	}
+
 	/// Create an offer for receiving async payments as an often-offline recipient.
 	///
-	/// Because we may be offline when the payer attempts to request an invoice, you MUST:
+	/// Instead of using this method, prefer to set [`UserConfig::paths_to_static_invoice_server`] and
+	/// retrieve the automatically built offer via [`Self::get_cached_async_receive_offer`].
+	///
+	/// If you want to build the [`StaticInvoice`] manually using this method instead, you MUST:
 	/// 1. Provide at least 1 [`BlindedMessagePath`] terminating at an always-online node that will
 	///    serve the [`StaticInvoice`] created from this offer on our behalf.
 	/// 2. Use [`Self::create_static_invoice_builder`] to create a [`StaticInvoice`] from this
@@ -10353,6 +10367,10 @@ where
 	/// Creates a [`StaticInvoiceBuilder`] from the corresponding [`Offer`] and [`Nonce`] that were
 	/// created via [`Self::create_async_receive_offer_builder`]. If `relative_expiry` is unset, the
 	/// invoice's expiry will default to [`STATIC_INVOICE_DEFAULT_RELATIVE_EXPIRY`].
+	///
+	/// Instead of using this method to manually build the invoice, prefer to set
+	/// [`UserConfig::paths_to_static_invoice_server`] and retrieve the automatically built offer via
+	/// [`Self::get_cached_async_receive_offer`].
 	#[cfg(async_payments)]
 	pub fn create_static_invoice_builder<'a>(
 		&self, offer: &'a Offer, offer_nonce: Nonce, relative_expiry: Option<Duration>