C++/WinRT: provide wrappers for authoring a DependencyProperty

[jaigak]

Consider providing wrappers to ease creating a DependencyProperty, member or attached, similar to those provided for regular properties and notifying properties.

[dunhor]

This was started with #472, but seems to have fizzled out

[sylveon]

We might wanna revisit that PR with C++26 reflection instead of macros, anyways.

[amtopel]

Anyone know why it fizzled out, and how we can get the proposal accepted? It would be a great addition.