[ENH] Handling HttpRequest Serialization in HTTP Endpoint Activity

## Enhancement Request

### Enhancement Overview

The "Result" output of the "HTTP Endpoint" activity can not be process in a variable for the moment, because the underlying object (a `DefaultHttpRequest`) can not be serialized when the activity execution entity is built.

### Proposed Enhancement

A `JsonConverter` could be added to handle the `HttpRequest` objects. The developers could also have a clean way to add some custom `JsonConverter`, for another exotic types, without reimplementing `IPayloadSerializer`.