Make formatter and unknown fields public in ProtoProducerMessage

[mieczkowski]

I propose to make formatter and unknown fields map more public in protoproducer.ProtoProducerMessage struct.

The reason is that I try to create flows enrichment in custom formatter.

My struct is:

type FlowMessage struct {
	*protoproducer.ProtoProducerMessage
	SrcCountry string `json:"src_country"`
	DstCountry string `json:"dst_country"`
	// other fields
}

In this scenario in method Format(data interface{}) ([]byte, []byte, error) I can fill additional fields without make a copy of original struct. But marshall to json needs to be done in three steps:

1. Marshal embedded struct to json with m.ProtoProducerMessage.MarshalJSON() to create unknownFields as fields in json and use renderers from config
2. Parse json into map, fill additional fields
3. Marshal map to json

With access to formatter from ProtoProducerMessage I could write more efficient json serializer with my fields.

I also need unknown fields for two reasons:

1. Custom json marshal (obvious ;-) ). If I will make more efficient json marshal function I will be happy to propose it in another PR,
2. I use expr to make some queries from flow, for example: FlowDirection == "12345" and DstAddr == IP("1.1.2.2")). FlowDirection is in UnknownFields, so I need to materialize them in my struct as map or fields.