help set windows icons

https://anthropicstudios.com/2021/01/05/setting-a-rust-windows-exe-icon/

This is Non-Trivial but it all seems reasonably within scope of cargo-dist and wouldn't be too hard to implement for someone sufficiently motivated. We already Do Icon Things and Grab Extra Toolchains with the cargo-wix msi subsystem. That said there's a fair amount of design work around like, config here (although a quick-n-dirty hack could sniff the cargo-wix config and reuse that).

Beyond config it's all about adding new Tools and new BuildSteps I guess.

Start with the linked Windows icon article and inspect the existing cargo-wix MSI subsystem, especially its icon handling and extra-toolchain logic. Map the proposed configuration, Tools, and BuildSteps before deciding scope; done means cargo-dist can support setting Windows executable icons through a defined configuration path.

Written by the indexing model from the issue text.