Implement iOS/macOS secondary ToolbarItems (#28185)

* Implement iOS secondary ToolbarItem

* Icon support & WIP

* PR feedback

* Update NavigationRenderer.cs

* Improved sample

* Code styling and cleanup

* Fix NRE

* Fix UI test

* [api] Fix public files

* Review feedback

* Fix Public API, hopefully

* And add back the one public API we need...

---------

Co-authored-by: Rui Marinho <me@ruimarinho.net>

Author: jfversluis

src/Controls/docs/Microsoft.Maui.Controls/ToolbarItem.xml

@@ -0,0 +1,82 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.Maui.Controls;
+
+namespace Maui.Controls.Sample.Pages;
+
+public partial class ToolbarPage
+{
+	ToolbarItem? _cachedSecondary3;
+
+	public Command ClickedCommand { get; }
+
+	public ToolbarPage()
+	{
+		InitializeComponent();
+
+		ClickedCommand = new Command(() =>
+		{
+			menuLabel.Text = "You clicked on ToolbarItem: Test Secondary (3)";
+		});
+
+		BindingContext = this;
+	}
+
+	void ItemClicked(object sender, EventArgs e)
+	{
+		if (sender is ToolbarItem tbi)
+		{
+			menuLabel.Text = $"You clicked on ToolbarItem: {tbi.Text}";
+		}
+	}
+
+	void Button_Clicked(object sender, EventArgs e)
+	{
+		secondary4.IsEnabled = !secondary4.IsEnabled;
+	}
+
+	void Button_Clicked1(object sender, EventArgs e)
+	{
+		primary1.IsEnabled = !primary1.IsEnabled;
+	}
+
+	void Button_Clicked2(object sender, EventArgs e)
+	{
+		// 5 second delay so you can have the menu open and see the change.
+		// However, the menu will close if change happens. There is no way around this.
+		Task.Delay(5000).ContinueWith(t =>
+		{
+			Dispatcher.Dispatch(() =>
+			{
+				secondary2.IsEnabled = !secondary2.IsEnabled;
+			});
+		});
+	}
+
+	void Button_Clicked3(object sender, EventArgs e)
+	{
+		secondary1.Text = secondary1.Text == "Test Secondary (1)" ? "Changed Text" : "Test Secondary (1)";
+	}
+
+	void Button_Clicked4(object sender, EventArgs e)
+	{
+		_cachedSecondary3 ??= secondary3;
+
+		if (ToolbarItems.Contains(_cachedSecondary3))
+		{
+			ToolbarItems.Remove(_cachedSecondary3);
+		}
+		else
+		{
+			ToolbarItems.Add(_cachedSecondary3);
+		}
+	}
+	
+	void Button_Clicked5(object sender, EventArgs e)
+	{
+		secondary3.Command = new Command(() =>
+		{
+			menuLabel.Text = "You clicked on ToolbarItem: Test Secondary (3) with changed Command";
+		});
+	}
+}