Put urls for service calls to Mail Service into a config file

issue in packagecontroller.cs

```
...
PackageServiceModel pkgService = await flurlPackageResponse.GetJsonAsync();
PackageResponseModel pkg = ServiceToResponseModelConverter.ConvertPackage(pkgService, await GetAllPersons(), await GetAllRooms());

Console.WriteLine("package route finished is " + pkg.RouteFinished);

if (pkg.RouteFinished)
{
//send email
await "https://mailservice20210603092014.azurewebsites.net/api/ArrivalMail?code=gYOUs9FO7WwwNXz2eSGtZM0AFxQl/RQvOJ4RF0uotwYLe7l/AIGGKg==".PostJsonAsync(pkg);
}
...
```

currently the urls for the calls to the serverless function mailservice is hardcoded

good practice would be to move the urls into a config file and take the value from the config file. Figure out a way to take the url from the config file. Note: in production, the url should be taken from GitHub secrets!

tip:
look at how the packageservice url has been set up as an example of how it's done