[Enhancement] Add Flyout Title and FlyoutIcon when FlyoutDisplayOptions="AsMultipleItems" is set.

### Description

When grouping flyout times in a menu, it would be nice if the "Title" and "Icon" properties (already present) were used as a header/title for the items underneath it.

I am basing this issue on Xamarin.Forms [#13343](https://github.com/xamarin/Xamarin.Forms/issues/13343)

### Public API Changes

The Title field is already present in the API; it is not used when FlyoutDisplayOptions="AsMultipleItems" is set.

```xaml

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### Intended Use-Case

#### Use-Case 1 (From the original Xamarin.Forms issue)
The flyout menu can be hard to read when there are a lot of items. Being able to group them with a header would be ideal. In the image below, adding "MyTitle" to the grouping would make it easier to find a menu item. My online banking app uses such an approach.

#### Use-Case 2
In this one, I wish to have a header called "Recommended Mechanics, Dealerships and Stores" with either nested or collapsible items. The routes and ContentTemplates are just placeholders

```xaml

```

An example of it potentially working could be similar to how WinUI uses NavItems

Start by tracing how FlyoutItem and FlyoutDisplayOptions="AsMultipleItems" are handled in .NET MAUI's flyout navigation. Review the existing Title and Icon properties and compare the requested grouping behavior with the linked Xamarin.Forms issue and WinUI example. Done means grouped flyout items display the parent title and icon consistently across supported platforms.

Written by the indexing model from the issue text.