Add [Optional] on optional params

[jevansaks]

There is a C# feature that if you have [Optional] on parameters that callers can omit those parameters and the compiler will automatically pass default for those parameters. This is almost like providing = default values on the parameters except that it also works for any parameter, not just trailing parameters.

What this means is that we can put [Optional] on any parameter that is optional which doesn't also have an out or ref modifier on it, and that will give callers a little more flexibility and communicate which parameters are optional / can be omitted.

Fixes #1580 , Fixes #652

[DoctorKrolic]

Can you please also add test for FwpmEngineOpen0? As per microsoft/win32metadata#2153 the first parameter in native overload didn't have [Optional] on it. It is not tagged [Reserved], but the native overload still lacks [Optional] attribute. This is either already fixed with your changes or needs some additional tweaks. Either way, it is a good test case verifying that a combination of [Reserved] and [Optional] still puts [Optional] in the native overload

[jevansaks]

I added a test for that function. In the latest win32metadata, that parameter is [Reserved] so it's always omitted on the friendly overload.

[DoctorKrolic]

I meant a test for the native overload. It is a good thing that this parameter is omitted from the friendly one, but the native should still have this parameter as [Optional], so it can be skipped when using named argiments

[jevansaks]

Oh, sure. Updated that test to check the native signature instead.