iOS ContactProviderExtension Support

### Description

The ContactProviderExtension protocol, introduced in iOS 18.0+, allows apps to provide contact items to the system-wide Contacts ecosystem. This functionality is crucial for applications that need to integrate deeply with the native Contacts framework, enabling users to manage, share, and retrieve contacts seamlessly across apps. Currently, the ContactProviderExtension is supported natively by iOS, but there is no direct support for it within .NET MAUI, which limits developers who wish to leverage this feature in cross-platform applications.

When implemented, the ContactProviderExtension in MAUI would allow developers to create app extensions that provide contact data to the Contacts system on iOS devices. This would involve using the ContactItemEnumerating and AppExtension protocols to enumerate and provide contacts from a custom data store, similar to how it's done in native iOS development.

### Public API Changes

https://developer.apple.com/documentation/contactprovider/contactproviderextension

### Intended Use-Case

https://developer.apple.com/documentation/contactprovider/contactproviderextension

Start with the linked Apple ContactProviderExtension documentation and review the ContactProviderExtension, ContactItemEnumerating, and AppExtension protocols named in the issue. Determine how these iOS 18 APIs should be exposed for .NET MAUI app extensions; done means MAUI developers can implement the extension and provide contact items to the system Contacts ecosystem.

Written by the indexing model from the issue text.