packer: add `buf_size` option #228

Job	Run time
Run test with Python 3.13-dev on ubuntu-latest	49s
Run test with Python 3.12 on ubuntu-latest	32s
Run test with Python 3.11 on ubuntu-latest	47s
Run test with Python 3.10 on ubuntu-latest	35s
Run test with Python 3.9 on ubuntu-latest	33s
Run test with Python 3.8 on ubuntu-latest	37s
Run test with Python 3.13-dev on windows-latest	2m 6s
Run test with Python 3.12 on windows-latest	1m 33s
Run test with Python 3.11 on windows-latest	1m 31s
Run test with Python 3.10 on windows-latest	1m 24s
Run test with Python 3.9 on windows-latest	1m 28s
Run test with Python 3.8 on windows-latest	1m 27s
Run test with Python 3.13-dev on macos-latest	47s
Run test with Python 3.12 on macos-latest	30s
Run test with Python 3.11 on macos-latest	27s
Run test with Python 3.10 on macos-latest	43s
Run test with Python 3.9 on macos-latest	45s
Run test with Python 3.8 on macos-latest	58s
	17m 32s

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