Add support for writing raw JSONs

[alexanderdean]

This would enable Snowplow ES Loader to go beyond pure-Snowplow usecases (similar to how Kinesis S3 already does).

This has a few advantages:

• Increases the utility of the Snowplow Elasticsearch Loader
• Increases the number of possible contributors to this project
• Allows us (Snowplow) to use this for getting customers' non-Snowplow JSON data into Elasticsearch, alongside their Snowplow events

[alexanderdean]

Let's get an estimate (in due course) for the scale of this work before deciding when to schedule...

[alexanderdean]

Let's do this in the next release.

@BenFradet can you suggest an extension to the existing configuration format for this ticket? So the options we want to support are:

1. The source stream is a Snowplow enriched event stream
2. The source stream is a Snowplow bad rows stream
3. The source stream is plain JSON