Pick multiple files with MediaPicker

[jfversluis]

Implement the ability to pick multiple files at once through MediaPicker.

I think what would make most sense to add a property like SelectionLimit to the MediaPickerOptions object that defaults to 1.

public class MediaPickerOptions
{
   /// <summary>
   /// Gets or sets the maximum count of media items to pick.
   /// The default value is 1.
   /// </summary>
   public int SelectionLimit { get; set; } = 1;
}

Q: Is there a maximum per platform?
Q: Is this functionality available on all platforms?
Q: Can we mix images and videos? On all platforms?

[AndreKraemer]

This would be super useful. Multiple clients asked us about multi selection!

[cvchris]

Same here, this feature would be really useful for our application

[ewerspej]

Yep, same here, we could use this in our client's app

[jfversluis]

Implemented with #30002