additionalParameters
Parses an optional Map<String, String> field from the JSON object.
Each value in the object must be a JSON string; if any value is a non-string type an JsonConfigError.InvalidType is thrown naming the offending nested key (e.g. "additionalParameters.badKey"), matching iOS behaviour.
Return
The parsed map, or null if the key is absent.
Parameters
key
The JSON key for the map field.