Add toolset_cuda conf for cmaketoolchain

[jcar87]

Changelog: Feature: Add configuration to specify desired CUDA Toolkit in CMakeToolchain for Visual Studio CMake generators.
Docs: conan-io/docs#3568

This allows to append the cuda=xxx key to CMAKE_GENERATOR_TOOLSET (docs) This is useful for a couple of cases:

• CUDA Toolkit is installed on a system location ("Program Files") and the user wants to select a specific version
• CUDA Toolkit is in a Conan recipe, and we want to define this in the package_info such that CMake will correctly use that toolkit for a CMake project on Windows with MSVC

Tests incoming!