[ntuple] Add RNTupleModel::GetMutableField #12946

Job	Run time
Windows 10 x64 Release	58m 8s
Windows 10 x86 Release	56m 8s
mac13 ARM64 LLVM_ENABLE_ASSERTIONS=On, builtin_zlib=ON	27m 27s
mac14 X64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	1h 50m 36s
fedora39 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	42m 8s
Upload Event Payload	3s
mac15 ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	26m 57s
alma8 LLVM_ENABLE_ASSERTIONS=On	43m 32s
mac-beta ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	27m 15s
alma9 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 22m 14s
ubuntu20 LLVM_ENABLE_ASSERTIONS=On	27m 22s
ubuntu22 imt=Off, LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 29m 40s
ubuntu2404 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 45m 1s
debian125 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	47m 54s
alma9 modules_off runtime_cxxmodules=Off	34m 36s
alma9 march_native CMAKE_BUILD_TYPE=RelWithDebInfo, CMAKE_CXX_FLAGS=-march=native, CMAKE_C_FLAGS=-march=native, fortran=OFF	45m 58s
alma9 arm64 CMAKE_BUILD_TYPE=RelWithDebInfo, ccache=ON	36m 33s
alma9-clang clang LLVM_ENABLE_ASSERTIONS=On, CMAKE_C_COMPILER=clang, CMAKE_CXX_COMPILER=clang++	35m 57s
ubuntu2404-cuda gpu	32m 8s
	15h 29m 37s

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