feat(wip): uniffi wrapper for payment preimage

In a bid to create a uniffi wrapper for PaymentPreimage,
we also create bindings-specific variants for the ffg types:
LightningBalance, PaymentKind, and Event, all of which have
fields that hold object/interface instances - PaymentPreimage,
ConfirmationStatus and ClosureReason.

We implement type conversion for the types created.

Unfortunately the build does not compile.
According to the uniffi docs:
"When you want to ... store object instances as fields in records,
 the underlying Rust code will need to work with Arc<T> directly,
 to ensure that the code behaves in the way that UniFFI expects."

Arc-ing the fields that hold PaymentPreimage, ConfirmationStatus,
& ClosureReason is not sufficient to circumvent the error
associated with objects not being supported in enum variants.

Author: enigbe

bindings/ldk_node.udl

@@ -803,6 +803,11 @@ interface Bolt12Invoice {
 	sequence<u8> encode();
 };
 
+interface PaymentPreimage {
+	[Throws=NodeError, Name=from_str]
+	constructor([ByRef] string preimage_str);
+};
+
 [Custom]
 typedef string Txid;
 
@@ -830,9 +835,6 @@ typedef string PaymentId;
 [Custom]
 typedef string PaymentHash;
 
-[Custom]
-typedef string PaymentPreimage;
-
 [Custom]
 typedef string PaymentSecret;