Add --sys-fn-ptrs for windows-sys fn ptr types

[uint-nate]

Potential solution for #3781

For reference, I used code built on this to generate the following bindings: https://github.com/uint-nate/pluginrust/blob/master/bindings/src/bindings.rs

        pub type EXPERIMENTAL_WebAuthNDecodeGetAssertionRequest = unsafe extern "system" fn(cbencoded : u32, pbencoded : *const u8, ppgetassertionrequest : *mut *mut _EXPERIMENTAL_WEBAUTHN_CTAPCBOR_GET_ASSERTION_REQUEST) -> windows_sys::core::HRESULT ;
        windows_link::link!("webauthn.dll" "system" fn EXPERIMENTAL_WebAuthNDecodeGetAssertionRequest(cbencoded : u32, pbencoded : *const u8, ppgetassertionrequest : *mut *mut _EXPERIMENTAL_WEBAUTHN_CTAPCBOR_GET_ASSERTION_REQUEST) -> windows_sys::core::HRESULT);

[uint-nate]

@microsoft-github-policy-service agree

(shouldn't be necessary since I'm a MSFT employee but 🤷)

[uint-nate]

@microsoft-github-policy-service agree company="Microsoft"

[uint-nate]

Updated so that if you specify --sys and --sys-fn-ptrs, then you'll get the function pointer types along side the function bindings.

Also attempted and hopefully successfully added a test for it. Not really familiar with the CI process here but I think the system yelled at me enough to get it mostly right?

This is my first Rust code that isn't just me playing around on toy projects, so happy to take any feedback around best practices that I may be ignorant of 😀.

[uint-nate]

https://github.com/uint-nate/pluginrust/blob/master/bindings/src/bindings.rs is updated to use this new binding code.

[uint-nate]

The current failures aren't coming from my code FWIW. If you want me to address them I can look into them though if that's the normal flow of things. Not sure if there was a library update or something.

[kennykerr]

That's a common problem as CI tests against Rust nightly builds as an early warning system. This PR is now blocked on #3789 where I address this.

[kennykerr]

Looks good - thanks!