Add MessageConverters for IncomingRabbitMQMessage

[Crustack]

Closes #3220

I looked at how e.g. MQTT messages are converted via e.g. StringMessageConverter and came up with the following changes:

• Removed the String/generic JSON deserialization from IncomingRabbitMQMessage.convertPayload
• Added MessageConverter implementations for deserialization to String, JsonObject, JsonArray to comply with current behaviour
• Added MessageConverter implemenation for deserialization to any type if a JsonMapping implementation is on the classpath
• Added fromJson(String str, Type type) to JsonMapping interface with implementations for jackson + jsonb (Vert.x does not seem to support mapping for a java.lang.reflect.Type ?)

[Crustack]

@cescoffier @ozangunalp any chance of getting a first review for this before christmas?
No pressure, I would just prefer this to not get lost between the years 🙂

[ozangunalp]

I don't think this will be merged before next year. But this, message converters and overall a common connector agnostic Serde is on my short list. So do not worry. We'll advance on it very shortly!