Add capability to shard message output based on values found in messages

Currently, the transcoder emits messages to output sources based on the message type specified in the schema. This would add a capability to further filter messages based on input dimensions observed, sharding them to separate outputs. For example, if you specify a list of `--shard_keys` fully qualified with the parent message type, it would manufacture a file/table/topic for each distinct value found in that dimension. So

```--shard_keys SnapshotFullRefreshTcpLongQty.SecurityID```

would produce and populate a separate persistent resource for each unique value of that dimension found in the input data.