Add WinRT.StubExe project and .targets to support WinRT activation for apps

[Sergio0694]

Depends on: dotnet/runtime#103504

This PR includes two changes:

• Adds a new WinRT.StubExe project to produce a native .exe that will invoke a custom Main from applications, which will leverage the new CustomNativeMain functionality from .NET 9 (see related PR there). This enables apps to both be executed normally, as well as act as implementation .dll for WinRT components (eg. for OOP background tasks).
• Some .targets changes to wire things up when publishing (configuring NativeAOT props, bundling the stub .exe, etc.)

Note

Opening as a draft as things are still very much up in the air at this point.

[jkotas]

I think you will need to enable a more options here to pass https://github.com/microsoft/binskim

[Sergio0694]

Spoke at length with @MichalStrehovsky, @jkoritzinsky and @AaronRobinsonMSFT, and I think the conclusion is that the simplest/best way to do this is to port over some code from DNNE and have a custom MSBuild task just invoke cl.exe to build/link a C program just forwarding to the custom main. This will keep things simple enough (and all the code we need is already there in DNNE), will let us have a stub .exe directly linking to the custom main, without the need for LoadLibrary at runtime or anything else, and it will also allow us to customize the .exe when needed in a single step, by just passing additional arguments to the linker (eg. /APPCONTAINER). Going to close this one 🙂