Add option to interleave messages to an output destination

[salsferrazza]

In lieu of sharding output destinations by message type, support interleaved output of all message types to a single destination file, topic, etc.

This has some implications to schema generation and most obviously would not work for BigQuery output types. It would also require that Pub/Sub topic schemas or Avro outputs be handled differently in this mode, perhaps using the Union option in Avro.