Windows - add computer certs

[LukeMauldin]

Current code in windows.rs only loads certificates from the user store: https://github.com/ctz/rustls-native-certs/blob/main/src/windows.rs#L19
In our use case, we push certificates group Group Policy into the computer store. Would also recommend querying schannel::cert_store::CertStore::open_local_machine to include computer certificates in the results.

[djc]

Would you mind submitting a PR for this?

[cpu]

I recommend folks with more nuanced requirements for Windows & local root certificates try rustls-platform-verifier. In addition to improved behaviour on Windows it allows more robust certificate validation on other platforms.

I believe there's limited interest in expanding the rustls-native-certs handling of Windows trust stores since the platform verifier crate offers a better experience across several dimensions.