Support generic types in the HttpRequestHandlingEndpointSupport

[artembilan]

See SO for more info: https://stackoverflow.com/questions/55142118/httprequesthandlingmessaginggateway-json-array-payload

Also consider to support the same functionality in the JsonToObjectTransformer.

The general problem that some HttpMessageConverter implementations are GenericHttpMessageConverter, e.g. AbstractJackson2HttpMessageConverter with of Type which can be extracted from already supported ResolvableType for the requestPayloadType.

The same ResolvableType should be applied for the JsonToObjectTransformer.

Since it is an omission with the current implementation and there is no easy way to workaround it we definitely need to consider to back port it to 5.1.x.