To retain lifetime per-target input/output counts for a stream processor, we will save this information in checkpoint. We already save operator stats in checkpoints, so this will just build on that.

1. Modify the OperatorStatsDoc IDL struct to contain fields for the new fields in the in-memory OperatorStats struct. We can use array<PerTargetStatsDoc>
2. When then need to modify the utility methods toOperatorStatsDoc and to OperatorStats to save and restore per target states
3. Make sure to add tests using commonTest!
4. Enable the feature flag, put things under isInternal