Fix serialization of SubscriptionExecutionResult for System.Text.Json (graphql-dotnet#2030)

Author: sungam3r

src/GraphQL.ApiTests/GraphQL.SystemTextJson.approved.txt

@@ -15,6 +15,7 @@ namespace GraphQL.SystemTextJson
     public class ExecutionResultJsonConverter : System.Text.Json.Serialization.JsonConverter<GraphQL.ExecutionResult>
     {
         public ExecutionResultJsonConverter(GraphQL.Execution.IErrorInfoProvider errorInfoProvider) { }
+        public override bool CanConvert(System.Type typeToConvert) { }
         public override GraphQL.ExecutionResult Read(ref System.Text.Json.Utf8JsonReader reader, System.Type typeToConvert, System.Text.Json.JsonSerializerOptions options) { }
         public override void Write(System.Text.Json.Utf8JsonWriter writer, GraphQL.ExecutionResult value, System.Text.Json.JsonSerializerOptions options) { }
     }

src/GraphQL.NewtonsoftJson/ExecutionResultJsonConverter.cs

@@ -109,6 +109,6 @@ private void WriteExtensions(ExecutionResult result, JsonWriter writer, JsonSeri
 
         public override bool CanRead => false;
 
-        public override bool CanConvert(Type objectType) => objectType == typeof(ExecutionResult);
+        public override bool CanConvert(Type objectType) => typeof(ExecutionResult).IsAssignableFrom(objectType);
     }
 }

src/GraphQL.SystemTextJson/ExecutionResultJsonConverter.cs

@@ -198,5 +198,7 @@ private static void WriteExtensions(Utf8JsonWriter writer, ExecutionResult resul
 
         public override ExecutionResult Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
             => throw new NotImplementedException();
+
+        public override bool CanConvert(Type typeToConvert) => typeof(ExecutionResult).IsAssignableFrom(typeToConvert);
     }
 }