Support for non utf-8 payloads

[cadubentzen]

Currently

nest-kinesis-producer/src/kinesis-producer/batch-kinesis-publisher.ts

Line 31 in 3e7b9dd

return Buffer.from(data, 'utf8');

assumes that the data provided is an UTF-8 string which is then converted to Buffer.

However when using libraries such as kinesis-aggregation, the data is actually a protobuf as a Buffer already.

In a downstream branch I managed to solve it but had to change KinesisEvent's signature, which breaks the public api.

As I'm not that experienced with TypeScript, maybe there is a nice way to accomplish this that I'm not aware. Any pointers welcome and I could send a PR 😉