Expose building a JSON payload event with raw bytes in the EventData class

[YoEight]

Hey @OliverJurkschat, 

Something is missing indeed but from our side, an oversight to be precise. We deprecated relying on JsonMapper on any public API, like the EventDataBuilder you mentioned. 

The appropriate method is available if you look at `EventDataBuilder` class directly: https://eventstore.github.io/EventStoreDB-Client-Java/com/eventstore/dbclient/EventDataBuilder.html#json(java.lang.String,byte%5B%5D)

With this method, because you can provide the raw bytes yourself, you can use what ever you like to build your JSON. We forgot to expose that method in the `EventData` class.

Originally posted by @YoEight in #256 (comment)