[executorch] Migrate extension/kernel_util to new namespace

[dbort]

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Migrate these headers to the new ::executorch::extension namespace. Add temporary aliases from the old ::torch::executor namespace so we can migrate users incrementally.

Also hide a good chunk of it in an internal namespace. And use a unique internal namespace because it defines very generic names like "size" that could conflict with other extensions.

Differential Revision: D60938934