layers: Add string_join utility
layers: Add string_join utility

Added utility modelled on script languague "join" command for
concatenating collections of strings (of various types).

Change-Id: Ia6ee7a144e7abc49a3356e9e20802706191947c1

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers