[SourceGenerator/Samples] Ability to add additional C# files to C# code display in samples

### Describe the bug

Sometimes we write auxiliary helpers, converters, etc... when writing a sample.

Currently our XAML/C# code display only shows the main page. So we either have to include those converters inline in the page, but if we want to use them across samples then we'd want to have them as separate files.

Therefore we should have another attribute or parameter to an existing attribute that allows us to point to other classes whose source code we should include for the C# tab.

### Steps to reproduce

```text
N/A
```

### Expected behavior

Able to include multiple C# source files to show under the source code display in a sample.

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [ ] 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 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)
- [ ] 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

Yes, but only if others can assist.