Organize `StringUtil.h` and `Utils.h` into `utils/` and `VkUtils.h` (#4123)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4123

Everything in `utils/` will take the `namespace vkcompute:::utils`; everything in `VkUtils.h` deals with the Vulkan API directly.
ghstack-source-id: 232367489
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Reviewed By: SS-JIA

Differential Revision: D59281548

fbshipit-source-id: 875aa9b639b420a09ee418bad82eb5955604e92c