Refactor quantization into a shared extension/llm/export/quant package (#21044) #12827

Job	Run time
Get changed files / get-changed-files	4s
CI run decision / decide	31s
test-metal-backend-modules / macos-job	10m 12s
test-metal-qwen35-moe-tiny / macos-job	8m 53s
test-executorch-metal-build / macos-job	5m 59s
export-model-metal-artifact	0s
test-model-metal-e2e	0s
	25m 39s

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