Add json schema validation script

[christophe-lunarg]

Thanks to this script added, any layer project can validate the layer manifest settings with this simple CMake target:

# Copy the configured manifest JSON to the build directory and validate it against layers_settings_schema.json
add_custom_command(TARGET ProfilesLayer POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy_if_different "${INTERMEDIATE_FILE}" "$<TARGET_FILE_DIR:ProfilesLayer>/${OUTPUT_FILE_FINAL_NAME}"
    COMMAND ${Python3_EXECUTABLE} "${VULKAN_UTILITY_LIBRARIES_INSTALL_DIR}/scripts/validate_json_manifest.py"
            "$<TARGET_FILE_DIR:ProfilesLayer>/${OUTPUT_FILE_FINAL_NAME}"
            "${VULKAN_UTILITY_LIBRARIES_INSTALL_DIR}/docs/layers_settings_schema.json"
            "${CMAKE_BINARY_DIR}"
    COMMENT "Validating ${OUTPUT_FILE_FINAL_NAME} against layers_settings_schema.json"
    VERBATIM
)

Used in KhronosGroup/Vulkan-Profiles#1001

And this is what it will look like when this PR will be merged:
KhronosGroup/Vulkan-Profiles#1002