Add a ShaderGen option to select the target MDL version #2368

Job	Run time
Build (Linux_GCC_9_Python37, ubuntu-20.04, gcc, 9, 3.7, -DMATERIALX_BUILD_SHARED_LIBS=ON)	13m 14s
Build (Linux_GCC_11_Python39, ubuntu-20.04, gcc, 11, 3.9, ON)	24m 24s
Build (Linux_GCC_12_Python311, ubuntu-22.04, gcc, 12, 3.11, ON)	19m 55s
Build (Linux_Clang_10_Python37, ubuntu-20.04, clang, 10, 3.7, -DMATERIALX_BUILD_SHARED_LIBS=ON, ON)	14m 8s
Build (Linux_Clang_13_Python39, ubuntu-22.04, clang, 13, 3.9, ON)	15m 41s
Build (Linux_Clang_14_Python311, ubuntu-22.04, clang, 14, 3.11, ON)	15m 42s
Build (Linux_Clang_14_DynamicAnalysis, ubuntu-22.04, clang, 14, None, -DMATERIALX_DYNAMIC_ANALYSI...	21m 34s
Build (MacOS_Xcode_11_Python37, macos-11, xcode, 11.7, -DMATERIALX_BUILD_SHARED_LIBS=ON, 3.7)	20m 4s
Build (MacOS_Xcode_13_Python39, macos-12, xcode, 13.4, 3.9)	27m 10s
Build (MacOS_Xcode_14_Python311, macos-13, xcode, 14.3, 3.11, ON)	19m 51s
Build (Windows_VS2019_Win32_Python37, windows-2019, x86, 3.7, -G "Visual Studio 16 2019" -A "Win3...	16m 44s
Build (Windows_VS2022_x64_Python39, windows-2022, x64, 3.9, -G "Visual Studio 17 2022" -A "x64")	18m 48s
Build (Windows_VS2022_x64_Python311, windows-2022, x64, 3.11, -G "Visual Studio 17 2022" -A "x64"...	19m 3s
	4h 6m 18s

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