Register custom Gson Type adapter

[mynecker]

It would be helpful to be able to register custom Gson-Type adapters [1] for serialization and deserialization of Json payloads.

Gson gson = new GsonBuilder().registerTypeAdapter(MyClass.class, new MyAdapter()).create();

[1] https://google.github.io/gson/apidocs/com/google/gson/TypeAdapter.html

[this]

@mynecker Currently we don't have any plans on implementing this. As a workaround, you can call Gson by yourself and then set the output JSON string to the response.

[inem88]

I found solution for custom format fields, for example OffsetDateTime. See #415 (comment)