Updated ReachFrequencyPrediction spec + new nested ReachFrequencyPredictionPausePeriod adobjects

[Just1B]

Checklist

• [X] I've read the Contributing Guidelines and the Code of Conduct
• [X] I've completed the Contributor License Agreement

Pull Request Details

Updated ReachFrequencyPrediction spec based on Graph Api Responses Observation ( no description available in Meta's documentation )