HtlcId type converter to fix json serialization

Newtonsoft.Json needs types used as key in dictionaries
to be convertible to string so we need to create a type
converter to HtlcId.

Author: aarani

src/DotNetLightning.Core/Utils/Primitives.fs

@@ -12,6 +12,7 @@ open DotNetLightning.Core.Utils.Extensions
 
 open ResultUtils
 open ResultUtils.Portability
+open System.ComponentModel
 
 [<AutoOpen>]
 module Primitives =
@@ -395,12 +396,29 @@ module Primitives =
 #if !NoDUsAsStructs
     [<Struct>]
 #endif
+    [<TypeConverter(typeof<HTLCIdToStringTypeConverter>)>]
     type HTLCId = | HTLCId of uint64 with
         static member Zero = HTLCId(0UL)
         member x.Value = let (HTLCId v) = x in v
 
         static member (+) (a: HTLCId, b: uint64) = (a.Value + b) |> HTLCId
 
+        override self.ToString() =
+            self.Value.ToString()
+
+    and private HTLCIdToStringTypeConverter() =
+        inherit TypeConverter()
+        override __.CanConvertFrom(_context, sourceType) =
+            sourceType = typeof<string>
+        override __.ConvertFrom(_context, _culture, value) =
+            match value with
+            | :? string as stringValue ->
+                stringValue
+                |> UInt64.Parse
+                |> HTLCId.HTLCId
+                |> box
+            | _ -> failwith "should not happen"
+
 #if !NoDUsAsStructs
     [<Struct>]
 #endif