Add support for batch in Message<List<Message<?>>> structure

[artembilan]

Right now batch support is present as a message of list of payloads and individual headers are stored as an item of list in some specific header.
The structure as Message<List<Message<?>>> makes it easier to navigate individual items in the batch with their specific header and so on.

The example is a result of Kinesis Binder on the consumer side:

[olegz]

From the s-c-function perspective this should be fairly easy to implement since it is really about message conversion and expectation of the converter.