Allow changing the target Windows SDK version

Projects compiled with the Visual Studio compiler can target different Windows SDK versions with support for different Windows versions. This can be set up using an additional parameter to the vcvarsall.bat script:

vcvarsall.bat [_architecture_] [_platform_type_] [_winsdk_version_] [*-vcvars_ver=*_vcversion_]

...

winsdk_version
Optionally specifies the version of the Windows SDK to use. By default, the latest installed Windows SDK is used. To specify the Windows SDK version, you can use a full Windows 10 SDK number such as 10.0.10240.0, or specify 8.1 to use the Windows 8.1 SDK.

https://docs.microsoft.com/en-us/cpp/build/building-on-the-command-line#vcvarsall-syntax

Currently, there's no way to change these additional parameters from Qt Creator. That means the highest installed Windows SDK is targeted by default, and there's no simple way to target a different SDK.

It would be great if the target SDK could be set in Qt Creator, either globally in the Kit/compiler settings, or (even better) per project.