Consolidate the evalResult for kafka, kinesis and pubsub logging

[zhouzhuojie]

It would be great to merge the evalResult struct for kafka and kinesis. It make future vendors for data recording easier.

https://github.com/checkr/flagr/blob/master/pkg/handler/data_recorder_kafka.go#L136
https://github.com/checkr/flagr/blob/master/pkg/handler/data_recorder_kinesis.go#L88

type kafkaMessageFrame struct {
	Payload   string `json:"payload"`
	Encrypted bool   `json:"encrypted"`
}

type kinesisMessageFrame struct {
	Payload   string `json:"payload"`
	Encrypted bool   `json:"encrypted"`
}

[zhouzhuojie]

Note, we also need to consolidate pubsub as well