Add API for building attribute getter free functions. #2777

Job	Run time
macos-11 appleclang13/C++17 py3.10 MacOS-11	11m 37s
macos-12 appleclang13/C++17 py3.10 MacOS-12	13m 57s
macos-13 appleclang14/C++17 py3.11 MacOS-13	11m 32s
Ubuntu Debug gcc7/C++14 llvm9 py2.7 oiio2.3 exr2.4 sse4 boost1.65 exr2.4	5m 1s
Ubuntu gcc10/C++17 llvm10 oiio-release boost1.65 exr2.5 avx2	3m 41s
Ubuntu latest releases gcc11/C++17 llvm13 boost1.71 exr3.1 py3.9 avx2 batch-b16avx512	14m 38s
Ubuntu bleeding edge gcc12/C++17 llvm14 oiio/ocio/exr/pybind-master boost1.71 py3.10 avx2 batch-b16avx512	3m 43s
Ubuntu clang14/C++17 llvm14 boost1.71 exr3.1 py3.8 avx2 batch-b16avx512	6m 12s
VFX2019 gcc6/C++14 llvm9 py2.7 boost1.66 exr2.3 oiio2.3 sse2	5m 46s
VFX2019 clang9/C++14 llvm9 oiio-release boost1.66 avx2 exr2.3 py2.7	3m 47s
VFX2019 GPU Cuda10 gcc6/C++14 llvm10 py2.7 boost-1.70 exr-2.3 OIIO-master avx2	3m 21s
VFX2020 gcc6/C++14 llvm10 py3.7 boost1.70 exr2.4 oiio2.2 sse4	7m 46s
VFX2021 gcc9/C++17 llvm11 py3.7 boost1.73 exr2.5 oiio2.3 avx2 batch-b8avx2	7m 20s
VFX2022 gcc9/C++17 llvm13 py3.9 boost1.76 exr3.1 oiio-rel avx2	6m 16s
VFX2022 clang12/C++17 llvm12 oiio-master boost1.73 exr3.1 py3.9 avx2 batch-avx512	5m 1s
VFX2022 icc/C++17 llvm14 py3.9 boost1.76 exr3.1 oiio-master avx2	28m 17s
VFX2022 icx/C++17 llvm14 py3.9 boost1.76 exr3.1 oiio2.3 avx2	8m 38s
VFX2023 gcc11/C++17 llvm15 py3.10 boost1.80 exr3.1 oiio-rel avx2	6m 45s
VFX2019 oldest everything gcc6/C++14 llvm9 py2.7 boost1.66 oiio2.3 no-simd exr2.3	5m 40s
VFX2022 sanitizers	8m 18s
VFX2022 clang-format	4m 48s
	2h 52m 4s

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