Feature Request: Implement missing AppNotificationBuilder APIs

### Describe the bug

`AppNotificationBuilder` is missing APIs that were available in UWP's `ToastContentBuilder`:
- `AdaptiveText`
- `AdaptiveGroup`
- `AdaptiveSubgroup`
- `AddVisualChild`
- ...

### Steps to reproduce the bug

In UWP I was able to do the following. Those APIs are not available for `AppNotificationBuilder`
```
var text = new AdaptiveText { HintWrap = true, Text = text, HintStyle = AdaptiveTextStyle.CaptionSubtle };
new ToastContentBuilder()
.AddVisualChild(new AdaptiveGroup { Children = { new AdaptiveSubgroup { Children = { text } } } });
```

It's also documented as not being available: https://learn.microsoft.com/en-us/windows/apps/design/shell/tiles-and-notifications/adaptive-interactive-toasts?tabs=appsdk#columns-and-text-elements

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

Windows App SDK 1.7.3: 1.7.250606001

### Packaging type

Packaged (MSIX)

### Windows version

Windows 11 version 24H2 (22621, October 2024 Update)

### IDE

Visual Studio 2022

### Additional context

_No response_

Start with AppNotificationBuilder and compare its available surface with UWP's ToastContentBuilder, focusing on AdaptiveText, AdaptiveGroup, AdaptiveSubgroup, and AddVisualChild. Review the linked adaptive interactive toasts documentation and define completion as supporting the listed APIs and the demonstrated nested text composition pattern.

Written by the indexing model from the issue text.