additionalProperties: false required at lower object levels

[annakrystalli]

As we've discovered in the past, if not specified explicitly, the default for objects is to allow additionalProperties. While we've addressed this at higher levels (e.g config level, output_types and target_metadata - we do allow additional properties in rounds and of course task ids) we haven't in lower level objects. This means that while we're removing optional as a valid property of output types in v4.0.0, the continued existence of optional properties is not causing a validation error.

We therefore need to add "additionalProperties": false to all lower level object specifications 🙄