Port AutoFocusBehavior Behavior

### Describe the bug

`AutoFocusBehavior` behavior class is missing in the original port.

References and related links:
- https://github.com/CommunityToolkit/Windows/releases/tag/v8.0.230907
> 🔥🐉 Note: Mixing and matching these new 8.0 packages with existing Windows Community Toolkit 7.x packages, especially UI controls, can lead to issues with the namespace changes and dependencies on common code (like the Visual Tree extensions). If there's something that we haven't yet ported but you need, [open a discussion](https://github.com/CommunityToolkit/Windows/discussions) to let us know or claim it and contribute!
- [AutoFocusBehavior](https://learn.microsoft.com/en-us/dotnet/api/communitytoolkit.winui.ui.behaviors.autofocusbehavior?view=win-comm-toolkit-dotnet-7.0) class documentation
- [AutoFocusBehavior.cs](https://github.com/CommunityToolkit/WindowsCommunityToolkit/blob/winui/CommunityToolkit.WinUI.UI.Behaviors/Focus/AutoFocusBehavior.cs) source code in v7

### Steps to reproduce

```text
N/A
```

### Expected behavior

N/A

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [X] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 10 21H2 (Build 19044)
- [ ] Windows 10 22H2 (Build 19045)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Windows 10, version 2104 (Build 20348)
- [ ] Windows 11, version 22H2 (Build 22000)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

### Help us help you

No, I'm unable to contribute a solution.

Beginne damit, den verlinkten Quellcode von v7 AutoFocusBehavior.cs und die Dokumentation seiner Klasse zu lesen, und ermittle anschließend das entsprechende WinUI 3-Behavior-Projekt in diesem Repository. Vergleiche die vorhandenen Behavior-Portierungen und Tests, sofern vorhanden, bevor du die fehlende Klasse hinzufügst; fertig ist die Aufgabe, wenn AutoFocusBehavior für die WinAppSDK / WinUI 3-Plattform verfügbar ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.