[Enhancement] iOS: Add support for BarButtonItem to Share API to set position of presentation controller

Description

There's already a property ShareRequestBase.PresentationSourceBounds to set a Rectangle to give the presentation controller used by UIActivityViewController on iPads a hint where to anchor the displayed view. It's directly applied to activityController.PopoverPresentationController.SourceRect.

I suggest a API change/addition to be able to pass a UIBarButtonItem instead to the presentation controller as there's no good way to find the SourceRect for a bar button item.

Was a feature request as https://github.com/xamarin/Essentials/issues/1939

Public API Changes

Adds a new property ShareRequestBase.PresentationOptions
Adds an interface IPresentationOptions
Adds a shared class SourceBoundsPresentationOptions
Adds a iOS class UIBarButtonItemPresentationOptions
Changes behavior of ShareRequestBase.PresentationSourceBounds to be a projection of new ShareRequestBase.PresentationOptions property. One may deprecated this property completely.

public abstract class ShareRequestBase
{
    // ...
    public IPresentationOptions PresentationOptions { get; set; }
    public Rectangle PresentationSourceBounds {
        get
        {
            if(PresentationOptions is SourceBoundsPresentationOptions bounds)
            {
                return bounds.Bounds;
            }
            else
            {
                return Rectangle.Empty;
            }
        }
        set => PresentationOptions = new SourceBoundsPresentationOptions(value);
    }
}
public interface IPresentationOptions {}
public sealed class SourceBoundsPresentationOptions : IPresentationOptions
{
    public SourceBoundsPresentationOptions(Rectangle bounds)
        => Bounds = bounds;
    public Rectangle Bounds { get; }
}
public sealed class UIBarButtonItemPresentationOptions : IPresentationOptions
{
    public UIBarButtonItemPresentationOptions(UIBarButtonItem barButtonItem)
        => BarButtonItem = barButtonItem;
    public UIBarButtonItem BarButtonItem { get; set; }
}

Intended Use-Case

There's no good way to find the SourceRect for a bar button item and though leverage the existing property ShareRequestBase.PresentationSourceBounds.

dotnet / maui