Consider moving `DateTimeOffsetExtension.MinutesSince()` logic to shared project

[CBielstein]

The DateTimeOffsetExtension.MinutesSince() method is used on the frontend to display how many minutes have passed since a packet was received. Similar logic is run on the backend to double check expiry. Consider either simply moving this extension to the shared project and reusing on the backend or adding it to the WeatherReport<T> class as a method.