Lazy-fetch the settings for extensions (#38844)

This PR stops us from synchronously initializing the settings page for every extension (including built-in's) on startup. That incurs a small penalty that really adds up the more extensions a user has.

Instead, we'll now only initialize the `CommandSettings` object when we first actually need it. 

From a relatively unscientific test, this saves approximately 10% on the initialization of builtin commands, and for my setup, it trims about 28% off extension initialization (across all built-in's / extensions):

branch | Built-in load (ms) | Extension load (ms) | %Δ builtin | %Δ extensions | 
-- | -- | -- | -- | -- |
main | 1455 | 6867.6 | | |
this PR | 1309.2 | 4919 | -10.02% | -28.37%

Closes #38321

src/modules/cmdpal/Microsoft.CmdPal.UI/Settings/ExtensionPage.xaml

@@ -113,7 +113,24 @@
                                     Visibility="{x:Bind ViewModel.HasSettings}" />
 
                                 <Frame x:Name="SettingsFrame" Visibility="{x:Bind ViewModel.HasSettings}">
-                                    <cmdpalUI:ContentPage ViewModel="{x:Bind ViewModel.SettingsPage, Mode=OneWay}" />
+
+                                    <controls:SwitchPresenter
+                                        HorizontalAlignment="Stretch"
+                                        TargetType="x:Boolean"
+                                        Value="{x:Bind ViewModel.LoadingSettings, Mode=OneWay}">
+                                        <controls:Case Value="True">
+                                            <ProgressRing
+                                                Width="36"
+                                                Height="36"
+                                                HorizontalAlignment="Center"
+                                                VerticalAlignment="Center"
+                                                IsIndeterminate="True" />
+                                        </controls:Case>
+                                        <controls:Case Value="False">
+                                            <cmdpalUI:ContentPage ViewModel="{x:Bind ViewModel.SettingsPage, Mode=OneWay}" />
+                                        </controls:Case>
+                                    </controls:SwitchPresenter>
+
                                 </Frame>
 
                                 <TextBlock