Customizable Content Dialog

## Summary
In xamarin it always pain to create custom dialog because it always rely on renderer.
In MAUI what if we can customize the dialog content with xaml, like UWP content dialog you just put xaml design in dialog content.

The issue names no implementation files, tests, or entry points. Start by reviewing MAUI's existing dialog APIs and how XAML content could be hosted, then compare the UWP ContentDialog model described in the request. Define the supported customization surface and cross-platform behavior before implementation; done should mean custom dialog content works consistently across the relevant MAUI targets.

Written by the indexing model from the issue text.