Resolves #2037

New CMake option VULKAN_HPP_GENERATOR_BUILD to control the creation of both VulkanHppGenerator and VideoHppGenerator targets, defaulting to ON as to not break anything.
Naming was based on VULKAN_HPP_SAMPLES_BUILD and VULKAN_HPP_TESTS_BUILD, could otherwise rename to VULKAN_HPP_BUILD_GENERATOR to fit VULKAN_HPP_RUN_GENERATOR naming scheme.

Also created the Vulkan-Hpp interface target, aliased as Vulkan::Hpp for those just looking to get the include directory. Naming is akin to the Vulkan-Headers with their Vulkan-Headers interface target, aliased as Vulkan::Headers.
The target is always created and it might make sense to use it in the other build targets as a unified way to include the generated headers? Otherwise it can be hidden behind a CMake option or function if necessary.

This PR would allow fetching a specific version of Vulkan headers using the following CMake syntax:

include(FetchContent)
FetchContent_Declare(vulkan-headers
    GIT_REPOSITORY "https://github.com/KhronosGroup/Vulkan-Headers.git"
    GIT_TAG "v1.4.303"
    GIT_SHALLOW ON
    OVERRIDE_FIND_PACKAGE
    EXCLUDE_FROM_ALL
    SYSTEM)
FetchContent_Declare(vulkan-hpp
    GIT_REPOSITORY "https://github.com/KhronosGroup/Vulkan-Hpp.git"
    GIT_TAG "v1.4.303"
    GIT_SHALLOW ON
    GIT_SUBMODULES ""
    OVERRIDE_FIND_PACKAGE
    EXCLUDE_FROM_ALL
    SYSTEM)
FetchContent_MakeAvailable(vulkan-headers vulkan-hpp)
target_link_libraries(${PROJECT_NAME} PRIVATE Vulkan::Headers Vulkan::Hpp)