[Popup] Add `ComplexPopup` to `CommunityToolkit.Maui.Sample`

[TheCodeTraveler]

Description of Change

This PR adds a more complex example of using a Popup that demonstrates the following:

1. Constructor injection, passing in a parameter using Dependency Injection via PopupService
2. Use the Popup.Opened event to trigger an action once the Popup appears
3. How to bind to PopupOptions
4. How to return a result

PR Checklist

• [x] Has samples (if omitted, state reason in description)
• [x] Rebased on top of main at time of PR
• [x] Changes adhere to coding standard
• [ ] Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

Let's add this sample to the Popup docs @bijington