Add ByteString converter

Author: Kristoffer Lerbæk Pedersen

src/Protobuf.System.Text.Json/InternalConverters/ByteStringConverter.cs

@@ -0,0 +1,25 @@
+using System.Text.Json;
+using Google.Protobuf;
+using Google.Protobuf.Reflection;
+using Protobuf.System.Text.Json.InternalConverters;
+
+internal class ByteStringConverter : InternalConverter
+{
+    public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
+    {
+        var base64String = ((ByteString)value).ToBase64();
+        writer.WriteStringValue(base64String);
+    }
+
+    public override void Read(ref Utf8JsonReader reader, IMessage obj, Type typeToConvert, JsonSerializerOptions options,
+        IFieldAccessor fieldAccessor)
+    {
+        var base64String = reader.GetString();
+        if (base64String is null)
+        {
+            return;
+        }
+        var value = ByteString.FromBase64(base64String);
+        fieldAccessor.SetValue(obj, value);
+    }
+}

src/Protobuf.System.Text.Json/InternalConverters/InternalConverterFactory.cs

@@ -1,4 +1,5 @@
 using System.Text.Json;
+using Google.Protobuf;
 
 namespace Protobuf.System.Text.Json.InternalConverters;
 
@@ -23,6 +24,11 @@ public static InternalConverter Create(FieldInfo fieldInfo, JsonSerializerOption
             var internalConverter = (InternalConverter) Activator.CreateInstance(typeof(ProtoEnumConverter), args: new object[] { fieldInfo.EnumType, jsonSerializerOptions.Encoder! })!;
             return internalConverter;
         }
+        else if (fieldInfo.FieldType == typeof(ByteString))
+        {
+            var internalConverter = new ByteStringConverter();
+            return internalConverter;
+        }
         else
         {
             var converterType = typeof(FieldConverter<>).MakeGenericType(fieldInfo.FieldType);