Add type-safe Event API

[chuwy]

Currently return type for most functions in EventTransformer is a String, which represents enriched event turned into a JSON object. While this is very unopionated, minimalistic and exactly what most Spark users need - it usually involves a lot of post-processing, e.g:

1. We always know that result is a JSON object, but in order to pull something out we need to do parse(result) and even parse(result).asInstanceOf[JObject]
2. Multiple fields in enriched event are required and thus must be there by definition, but we need to access them carefully: parsedJson.map("event_id").getOrElse(throw new RuntimeException("event_id is not present in enriched event")
3. To get list of shredded types we have a separate function jsonifyWithInventory.

I think all this information should be in one typesafe container with asString helper function, something like:

case class EnrichedEvent(json: JObject, shreddedTypes: Set[IgluUri], collectorTstamp: Instant, eventId: UUID) {
  lazy val asString: String = compact(json)
}

This will also give us #43 for free as this is a matter of one more asStringWithoutNulls function (but with better name).

[chuwy]

Will be very useful for Snowflake Transformer and almost vital for BigQuery Loader.

[chuwy]

Actually, this is vital for any Loader (including RDB Shredder) and for atomic events refactoring.

[chuwy]

Related: snowplow/iglu-central#778, snowplow/snowplow-rdb-loader#103