[ntuple] Add RNTupleModel::GetMutableField #12908

Job	Run time
mac13 ARM64 LLVM_ENABLE_ASSERTIONS=On, builtin_zlib=ON	29m 0s
mac14 X64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	2h 34m 9s
Windows 10 x64 Release	55m 8s
mac15 ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	24m 4s
Windows 10 x86 Release	1h 25m 19s
mac-beta ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	28m 45s
Upload Event Payload	5s
fedora39 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	44m 46s
alma8 LLVM_ENABLE_ASSERTIONS=On	40m 38s
alma9 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 0m 48s
ubuntu20 LLVM_ENABLE_ASSERTIONS=On	42m 18s
ubuntu22 imt=Off, LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 34m 56s
ubuntu2404 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 11m 57s
debian125 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	42m 14s
alma9 modules_off runtime_cxxmodules=Off	34m 13s
alma9 march_native CMAKE_BUILD_TYPE=RelWithDebInfo, CMAKE_CXX_FLAGS=-march=native, CMAKE_C_FLAGS=-march=native, fortran=OFF	38m 20s
alma9 arm64 CMAKE_BUILD_TYPE=RelWithDebInfo, ccache=ON	35m 45s
alma9-clang clang LLVM_ENABLE_ASSERTIONS=On, CMAKE_C_COMPILER=clang, CMAKE_CXX_COMPILER=clang++	27m 21s
ubuntu2404-cuda gpu	23m 59s
	15h 33m 45s

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