Allow JsonSerializerOptions parameters for user-defined and inputs and hardcode everything else to the source generator. (#182)

* Allow user-defined JsonSerializerOptions for user-defined and inputs and hardcode everything else to the source generator.

* Add validation for user-defined JsonSerializerOptions.

* Remove out of date comment.

* Address feedback and add tests

src/ModelContextProtocol/Protocol/Messages/JsonRpcResponse.cs

@@ -1,4 +1,4 @@
-
+using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol.Messages;
@@ -23,5 +23,5 @@ public record JsonRpcResponse : IJsonRpcMessageWithId
     /// The result of the method invocation.
     /// </summary>
     [JsonPropertyName("result")]
-    public required object? Result { get; init; }
+    public required JsonNode? Result { get; init; }
 }