Add MarshalJSON to kinesis event

[matejsladek]

Add MarshalJSON function to correctly interpreting timestamp of kinesis event

[fsenart]

Hi @matejsladek,

First of all, thank you very much for contributing to the project.

Can you please explain me why the MarshalJSON is needed for the timestamp private type?
We use the timestamp type only during the unmarshaling phase along with type aliasing to correctly decode the custom AWS time format from the raw kinesis event. After what the final public Record type has a usual time.Time ApproximateArrivalTimestamp and we do not need any other trick to handle its marshaling.

https://github.com/eawsy/aws-lambda-go-event/blob/master/service/lambda/runtime/event/kinesisstreamsevt/definition.go#L39