`SegmentedItem` should expose `ContentHolder`'s `Margin`

### Describe the bug

As `DefaultSegmentedItemStyle`
https://github.com/CommunityToolkit/Windows/blob/dec350be1dc1d7d9dda734f0de824d4f030aac35/components/Segmented/src/SegmentedItem/SegmentedItem.xaml#L627-L632

and `PivotSegmentedItemStyle`
https://github.com/CommunityToolkit/Windows/blob/dec350be1dc1d7d9dda734f0de824d4f030aac35/components/Segmented/src/SegmentedItem/SegmentedItem.xaml#L861-L865

the `ContentHolder` hardcoded it's `Margin`, but

https://github.com/CommunityToolkit/Windows/blob/dec350be1dc1d7d9dda734f0de824d4f030aac35/components/Segmented/src/SegmentedItem/SegmentedItem.xaml#L1061-L1066

`ButtonSegmentedItemStyle` using `Padding` property which come from `ButtonItemPadding` resource

### Expected behavior

Add a new Thickness resource like `DefaultSegmentedItemContentPadding`, so that we can alter the content padding easily.

### 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)
- [X] 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)
- [X] Windows 11, version 22H2 (Build 22000)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

2022

### Visual Studio Build Number

_No response_

### Device form factor

Desktop

### Additional context

_No response_

### Help us help you

Yes, I'd like to be assigned to work on this item.

Start in components/Segmented/src/SegmentedItem/SegmentedItem.xaml at the linked DefaultSegmentedItemStyle, PivotSegmentedItemStyle, and ButtonSegmentedItemStyle sections. Compare the ContentHolder margin with the ButtonItemPadding resource usage. Done means the content spacing can be customized through a new Thickness resource such as DefaultSegmentedItemContentPadding.

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