Add `DataLoadingTemplate`

[lostluna1]

Hope there is a template that can set the animation when the TableView is in the data loading state, for example:

<DataLoadingTemplate>
    <DataTemplate>
        <ProgressBar/>
    </DataTemplate>
</DataLoadingTemplate>

I think this feature can better indicate the current state of the TableView

[lostluna1]

        <tv:TableView
            Grid.Row="1"
            AutoGenerateColumns="False"
            ItemsSource="{x:Bind ViewModel.TestModels}">
            <tv:TableView.Template>
                <ControlTemplate>
                    <ProgressRing></ProgressRing>
                </ControlTemplate>
            </tv:TableView.Template>
            <tv:TableView.Columns>
                <tv:TableViewTextColumn
                    Width="*"
                    Binding="{Binding Name}"
                    Header="Name" />
            </tv:TableView.Columns>
        </tv:TableView>

Currently setting the TableView's Template directly will overwrite the entire table

[bartazalbln]

Great idea!
Now I use SwitchPresenter from CommunityToolkit but I add also "no items" state. Having this all built-in would be great feature and will save extra work each time.