implement $additionalItems

[yanick]

The additionalItems keyword controls whether it’s valid to have additional items in the array beyond what is defined in items. Here, we’ll reuse the example schema above, but set additionalItems to false, which has the effect of disallowing extra items in the array.

The additionalItems keyword may also be a schema to validate against every additional item in the array. In that case, we could say that additional items are allowed, as long as they are all strings:

https://json-schema.org/understanding-json-schema/reference/array.html?highlight=additionalitems