[Enhancement] Shell footer template in lieu of WinUI NavigationView.FooterMenuItem's

### Description

I've been struggling - between MAUI being unfinished and myself - to mimic NavigationView's footer menu items. I feel like that should take over Shell's FlyoutFooterTemplate. Shell MenuItem's should be NavigationView.FooterMenuItem's. Preview 14 has been absolutely the biggest and best change IMO to the overall UI so far. I absolutely love it. Shell acts almost exactly how I have my WinUI applications. Which is a NavigationView in Left mode with Top mode NavigationView's for tabs inside the Page's I navigate to. Works out absolutely wonderful. HOWEVER, I feel like footer template's are extremely overrated, and NavigationView.FooterMenuItem's are extremely underrated, so I was wondering, is there a plan to align Shell with NavigationView.FooterMenuItem's? I think that would be huge.

### Public API Changes

```xaml

or

or

```

### Intended Use-Case

Do people really take advantage of Shell.FlyoutFooter/FlyoutFooterTemplate? Seems really unnecessary IMO. I'd rather have FlyoutItem's(NavigationViewItem's) pinned to the bottom. Shell already has MenuItem's. Why not just take whatever MenuItem's are specified, convert them to NavigationViewItem's for NavigationView.FooterMenuItem's, and toss Shell.FlyoutFooter/FlyoutFooterTemplate right into the trash, allow ShellContent/FlyoutItem's inside Shell.FlyoutFooter, or offer both the FlyoutFooterTemplate - to allow someone to completely put whatever inside of it - and FlyoutItem/MenuItem's inside of FlyoutFooter.

Start by reviewing Shell's FlyoutFooter and FlyoutFooterTemplate APIs alongside WinUI NavigationView.FooterMenuItem and Shell MenuItem behavior. Define which XAML forms should be supported and whether existing footer-template behavior remains; done means the resulting API and navigation behavior are specified consistently across the affected platforms.

Written by the indexing model from the issue text.