Move sentencepiece to extension/llm/third-party (#4388) #2036

Job	Run time
build-llm-demo (bpe) / linux-job	29m 59s
build-llm-demo (tiktoken) / linux-job	32m 42s
upload-artifacts	1m 42s
test-llama-app (bpe) / mobile-job (android)	5m 2s
	1h 9m 25s

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