Add support for ExtensionKit App Extensions with extensionkit_extension = True

[maustinstar]

Related issue: #1943

Plugin App Extensions

Before iOS 16.0, App Extensions were a product type com.apple.product-type.app-extension, build artifacts were installed into the app’s PlugIns directory, and the bundle’s Info.plist properties are defined in an NSExtension dictionary.

ExtensionKit App Extensions

iOS 16, macOS 13, tvOS 16, and watchOS 9, introduced ExtensionKit extensions for newer extensions like the Background Asset Downloader, App Intents, and even third-party-defined extension points.

Distinct from the plugin-type app extensions, ExtensionKit extensions have a product type com.apple.product-type.extensionkit-extension, build artifacts are installed into the app’s Extensions directory, and the bundle’s Info.plist properties are defined in an EXAppExtensionAttributes dictionary.

Architecture Decisions

I noticed the watchos_extension has an attribute to change the product type and linking flags. It's used by setting application_extension = True on watchos_extension. This PR follows a similar pattern by setting extensionkit_extension = True to update the product type and bundle install location.

Alternatively, I considered introducing a separate rule for ExtensionKit app extensions, but this seems to make existing extension rules lose generalizability

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[BalestraPatrick]

Looks like we have this in the upstream branch now: 3eb01c3

Unclear yet how the implementation differs.