Add Offer wrapper for FFI bindings

Implement Offer struct in ffi/types.rs to provide a wrapper around LDK's Offer for cross-language bindings.

Modified payment handling in bolt12.rs to:
- Support both native and FFI-compatible types via type aliasing
- Implement conditional compilation for transparent FFI support
- Update payment functions to handle wrapped types

Added testing to verify that properties are preserved when wrapping/unwrapping between native and FFI types.

Author: alexanderwiederin

bindings/ldk_node.udl

@@ -736,6 +736,29 @@ interface Bolt11Invoice {
 	PublicKey recover_payee_pub_key();
 };
 
+[Enum]
+interface OfferAmount {
+	Bitcoin(u64 amount_msats);
+	Currency(string iso4217_code, u64 amount);
+};
+
+interface Offer {
+	[Throws=NodeError, Name=from_str]
+	constructor([ByRef] string offer_str);
+	OfferId id();
+	boolean is_expired();
+	string? description();
+	string? issuer();
+	OfferAmount? amount();
+	boolean is_valid_quantity(u64 quantity);
+	boolean expects_quantity();
+	boolean supports_chain(Network chain);
+	sequence<Network> chains();
+	sequence<u8>? metadata();
+	u64? absolute_expiry_seconds();
+	PublicKey? issuer_signing_pubkey();
+};
+
 [Custom]
 typedef string Txid;
 
@@ -754,9 +777,6 @@ typedef string NodeId;
 [Custom]
 typedef string Address;
 
-[Custom]
-typedef string Offer;
-
 [Custom]
 typedef string Refund;
 

src/ffi/conversions.rs

@@ -1,12 +1,13 @@
 mod uniffi_impl {
 	use std::sync::Arc;
 
+	use lightning::offers::offer::Offer as LdkOffer;
 	use lightning_invoice::{
 		Bolt11Invoice as LdkBolt11Invoice, Bolt11InvoiceDescription as LdkBolt11InvoiceDescription,
 	};
 
 	use crate::error::Error;
-	use crate::ffi::{Bolt11Invoice, Bolt11InvoiceDescription};
+	use crate::ffi::{Bolt11Invoice, Bolt11InvoiceDescription, Offer};
 
 	pub trait UniffiType {
 		type LdkType;
@@ -50,6 +51,20 @@ mod uniffi_impl {
 		}
 	}
 
+	impl UniffiType for Arc<Offer> {
+		type LdkType = LdkOffer;
+
+		fn from_ldk(ldk_value: Self::LdkType) -> Self {
+			Arc::new(Offer { inner: ldk_value })
+		}
+	}
+
+	impl UniffiConversionType for Arc<Offer> {
+		fn as_ldk(&self) -> Self::LdkType {
+			self.inner.clone()
+		}
+	}
+
 	pub fn maybe_convert<T: UniffiConversionType>(value: &T) -> T::LdkType {
 		value.as_ldk()
 	}