[Enhancement] Add UpdateMode for picker cells to have the value updated immediately

[paya-cz]

Summary

Picker cells update their value once the picker overlay is submitted. Xamarin.Forms actually comes with an iOS-specific flag that can be set up on pickers to update the value immediately:
https://docs.microsoft.com/en-us/xamarin/xamarin-forms/platform/ios/picker-selection

Consider adding a flag like this so the value of the cell is updated as the user spins the picker.

API Changes

The source code for the Xamarin.Forms implementation is available here:

https://github.com/xamarin/Xamarin.Forms/blob/bd31e1e9fc8b2f9ad94cc99e0c7ab058174821f3/Xamarin.Forms.Core/PlatformConfiguration/iOSSpecific/Picker.cs
https://github.com/xamarin/Xamarin.Forms/blob/1be09cb36dd9283361c92072f105cec1bc2bd08c/Xamarin.Forms.Platform.iOS/Renderers/PickerRenderer.cs#L362