Add a toMap data type converter to ProtobufDataFormat

Currently the ProtobufDataFormat is doing a good job of converting a map to Protobuf Message without relying from other components converting the message to Protobuf message explicitly when marshalling message. However, when unmarshaling an exchange, the data format output from the unmarshaling process is either JSON or Proto Message. However, it will be also nice if it can output Map format.
An easy way that I could think of is to add a DataTypeConverter with @converter annotation that will convert the data in case we do something like this exchange.getMessage().getBody(Map.class).