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

[salsferrazza]

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.