Use std::variant in ManifestYamlPopulator

[msftrubengu]

This PR uses std::variant instead of member pointers to manifest structs to be references used by the processing functions. std::variant contains pointers to manifest structs to be populated and is passed to the FieldProcessInfo.ProcessFunc. Each processing function is responsible of calling std::get or std::get_if with the correct pointer type. ValidateAndProcessFields takes the std::variant and pass it to the processing functions.

Processing functions that required the member types now take a pointer to the manifest struct. Some of these can go in the anonymous namespace, but I didn't want to add more moves.

The change will make implementing shadow manifests easier.

[JohnMcPMS]

This seems fragile, I would really prefer to avoid a capture. But I'm guessing it isn't nearly as easy to do as it is to say.

[msftrubengu]

I tried splitting GetRootFieldProcessInfo and calling ValidateAndProcessFields with root, root installer and root localization separately. but that will result in several ManifestError::FieldUnknown warning. I don't want to deal with looking at the errors and remove them.

I removed the capture, but it still needs to any_cast to manifest or installer/localization. I'm not sure if the capture is better than this try catch approach.

[JohnMcPMS]

I thought the point of this was to get rid of the this capture.

[msftrubengu]

I can't get rid of it in all of them :(

[JohnMcPMS]

You are going to hate me, but I think std::variant might be the better approach. I don't think it is reliant on typeid, which we want to avoid due to size bloat. It will work the same way, except that you have to up front define the set of pointer types you would accept.