Add templates for app extensions

We need templates for app extensions.

Pending: list all the app extensions by estimated usage to have a prioritized list of templates to implement.

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# List of templates (unsorted)

## iOS

* [ ] Audio Unit (com.apple.AudioUnit)

We don't have any existing templates for this extension.

* [ ] Audio Unit (com.apple.AudioUnit-UI)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/ActionExtension

* [ ] Authentication Services (com.apple.authentication-services-credential-provider-ui)

We don't have any existing templates for this extension.

* [ ] Call Directory (com.apple.callkit.call-directory)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/CallDirectoryExtension

* [ ] File Provider (com.apple.fileprovider-nonui)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/DocumentPickerFileProviderExtension

* [ ] Document Picker (com.apple.fileprovider-ui)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/DocumentPickerExtension

* [ ] Intents (com.apple.intents-service)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/IntentsExtension

* [ ] Intents UI (com.apple.intents-ui-service)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/IntentsUIExtension

* [ ] Custom Keyboard (com.apple.keyboard-service)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/CustomKeyboardExtension

* [ ] Message (com.apple.message-payload-provider)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/MessageExtension

* [ ] Network Extension (com.apple.networkextension.packet-tunnel)

We don't have any existing templates (although there have been bug reports
about these extensions, so some people are creating them).

* [ ] Photo Editing (com.apple.photo-editing)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/PhotoEditingExtension

* [ ] Content Blocker (com.apple.Safari.content-blocker)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/ContentBlockerExtension

* [ ] Shared Links (com.apple.Safari.sharedlinks-service)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/SharedLinksExtension

* [ ] com.apple.services

We don't have any existing templates for this extension.

* [ ] Share (com.apple.share-services)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/ShareExtension

* [ ] Spotlight Index (com.apple.spotlight.index)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/SpotlightIndexExtension

* [ ] Action Extension (com.apple.ui-services)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/ActionExtension

* [x] Notification Content (com.apple.usernotifications.content-extension)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/NotificationContentExtension
* https://github.com/xamarin/xamarin-macios/pull/21886

* [x] Notification Service (com.apple.usernotifications.service)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/NotificationServiceExtension
* https://github.com/xamarin/xamarin-macios/pull/21886

* [ ] (com.apple.watchkit)

N/A to .NET

* [ ] Today (com.apple.widget-extension)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/TodayExtension

## tvOS

* [ ] Broadcast Upload / UI (com.apple.broadcast-services)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/BroadcastUIExtension
* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.IPhone/MonoDevelop.IPhone/templates/Xamarin.iOS/Project/BroadcastUploadExtension

* [ ] (com.apple.tv-services)

* [ ] Top Shelf Extension (com.apple.tv-top-shelf)

* Working sample project: https://github.com/user-attachments/files/22771410/TvShelfBug-DotNetWorking.zip (from https://github.com/dotnet/macios/issues/13260#issuecomment-3381431996), which can be used as a starting point for the template.

## macOS

* [ ] FinderSyncExtension (com.apple.FinderSync)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.MonoMac/MonoDevelop.MonoMac/templates/XamMac2/Projects/FinderSyncExtension

* [ ] ShareExtension (com.apple.share-services)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.MonoMac/MonoDevelop.MonoMac/templates/XamMac2/Projects/ShareExtension

* [ ] TodayExtension (com.apple.widget-extension)

* https://github.com/xamarin/vsmac-xamarin-extensions/tree/47507384987d5522983bce0d92afb799b96b5a96/MonoDevelop.MonoMac/MonoDevelop.MonoMac/templates/XamMac2/Projects/TodayExtension

* [ ] Network Extension (com.apple.networkextension.packet-tunnel)

We don't have any existing templates (although there have been bug reports
about these extensions, so some people are creating them).

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Ref: https://github.com/xamarin/xamarin-macios/issues/10747