Proposal: Open source / make publicly available for `ReunionCaptionControlsWindow`

# Proposal:
Open source `ReunionCaptionControlsWindow` that is used when `ExtendsContentIntoTitleBar` is set to `true`, or make this class publicly available, or add a method, something like `AttachToHwnd(hwnd)`

## Summary
I found it quite handy for modernize a standard win32 window utilizing `AppWindowTitleBar.ExtendsContentIntoTitleBar`.
My use case is putting a webview2 into a blank win32 window (created simply with `CreateWindow` ), then `ExtendsContentIntoTitleBar` with it. (Don't ask me why I wouldn't do that with a winui3 window instead, the webview2 control in winui3 is completely unusable to me)
The caption button (that is a child window called `ReunionCaptionControlsWindow`) is nicely kept in front of the webview child hwnd, the dwm accent color border is nicely kept, and the hit testing is correctly handled (Webview content is shown through the caption bar area, but moving the window and hovering on the caption buttons are correctly routed to the caption buttons, instead of the webview window).

This is my current usage:

## Rationale
- With this proposal, we can more easily modernize a win32 window.
- Open sourcing it will allow developers more easily customize the caption buttons, (like I need to modify the height, other than the simple `Standard` / `Tall` options)

## Scope

| Capability | Priority |
| :---------- | :------- |
| Open source `ReunionCaptionControlsWindow`| Must |
| Allowing creating an instance of `ReunionCaptionControlsWindow` manually| Nice to have |
| Add customization methods to `ReunionCaptionControlsWindow`| Nice to have |

## Important Notes

Examples:
```cpp
ReunionCaptionControlsWindow captionButtonWindow;
captionButtonWindow.Height(56); //56 dip height
SetParent(captionButtonWindow.getHwnd(), parentHwnd);
```
or
```cpp
ReunionCaptionControlsWindow captionButtonWindow;
captionButtonWindow.Height(56); //56 dip height
captionButtonWindow.AttachToHwnd(parentHwnd);
```
## Open Questions

Start by locating the internal ReunionCaptionControlsWindow used with AppWindowTitleBar.ExtendsContentIntoTitleBar and review how it is attached to a Win32 window. Clarify whether the intended outcome is open sourcing the class, exposing it publicly, or adding an AttachToHwnd-style API. Done requires an agreed public scope that supports the stated caption-button customization needs.

Written by the indexing model from the issue text.