Add ability to extract time data from forecast and forecast hourly

[kamiyo]

Thinking ahead to home-assistant features, we should think about exposing the time properties for forecasts.

In the station I use, these are the relevant properties we can consider:

{
    "generatedAt": "2024-07-12T01:54:55+00:00",
    "updateTime": "2024-07-11T22:56:35+00:00",
    "validTimes": "2024-07-11T16:00:00+00:00/P7DT12H",
}

I suppose updateTime is the relevant one, since that is when the data was updated. But, I'm happy to implement the below logic for the other ones as well.

The way I see it working is making two new properties in SimpleNWS: _forecast_updated_time and _forecast_hourly_updated_time, which will get populated with the above updateTime within get_gridpoints_forecast() and get_gridpoints_forecast_hourly(). Then, we can add public accessors for the properties. This shouldn't break existing behavior. Let me know if this makes sense.

I'm happy to make a PR.