Create new encapsulation class to contain Alert and other object types

[ameihm0912]

The output path of the pipelines is currently limited to ingestion of Alert objects.

This makes it difficult to persist other types of data from the pipeline that are not neccessarily alerts but we may want to write to BigQuery. This is discussed a bit in #320.

The intent would be to modify the output path to consume this new container event, which could contain an alert or some other type of object (like a source summary, etc). Alert objects would be handled the way way they currently are once the encapsulation is stripped, but this would provide a means to produce other types of pipeline output.