[SizerBase] No event after sizing has occurred (keyboard)

### Describe the bug

I noticed that it's hard to understand when the sizer has finished being manipulated (especially keyboard). It should have an event that signals the manipulation (via mouse or keyboard) has actually finished.

### Steps to reproduce

```text
This works in the sample app as they use the controls directly, but if you're trying to respond to the change in another place (like DataTable) then it becomes more complex.
```

### Expected behavior

Ability to know when the sizer has finished sizing

### Screenshots

_No response_

### Code Platform

- [x] 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)
- [ ] Windows 11 22H2 (Build 22621)
- [ ] Windows 11 23H2 (Build 22631)
- [ ] Windows 11 24H2 (Build 26100)
- [ ] Windows 11 25H2 (Build 26200)
- [ ] Windows 11 26H1 (Build 28000)
- [ ] 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)
- [ ] Windows 11, version 24H2 (Build 26100)
- [ ] Windows 11, version 25H2 (Build 26200)
- [ ] Windows 11, version 26H1 (Build 28000)
- [ ] 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_

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