Button Horizontal and Vertical TextAlignment

### Description

I would love button supports HorizontalTextAlignment (and Vertical) as Label does.

### Public API Changes

```xaml

```

### Intended Use-Case

I had need the handler in android and i made it:

```csharp
protected override void ConnectHandler(MaterialButton platformView)
{
base.ConnectHandler(platformView);

OnTextAlignmentPropertyChanged();
}

public override void UpdateValue(string property)
{
base.UpdateValue(property);
if (property == ButtonControl.HorizontalTextAlignmentProperty.PropertyName || property == ButtonControl.VerticalTextAlignmentProperty.PropertyName)
OnTextAlignmentPropertyChanged();
}

private void OnTextAlignmentPropertyChanged()
{
if (VirtualView is ButtonControl buttonControl)
{
GravityFlag horizontalFlag = buttonControl.HorizontalTextAlignment switch
{
TextAlignment.Start => GravityFlag.Left,
TextAlignment.Center => GravityFlag.CenterHorizontal,
TextAlignment.End => GravityFlag.Right,
_ => GravityFlag.Center
};
GravityFlag verticalFlag = buttonControl.VerticalTextAlignment switch
{
TextAlignment.Start => GravityFlag.Top,
TextAlignment.Center => GravityFlag.CenterVertical,
TextAlignment.End => GravityFlag.Bottom,
_ => GravityFlag.Center
};
PlatformView.Gravity = horizontalFlag | verticalFlag;
}
}
```

Start at ButtonControl and compare its API with the Label text-alignment implementation, then inspect the platform button handlers, including the Android handler described in the issue. The work is complete when the public XAML properties are supported and horizontal and vertical alignment produce the requested button text positioning across the relevant platforms.

Written by the indexing model from the issue text.