This CL contains no functional change, it is a pure splitting of the existing files.
I elected to move the C++ kernel files to `cpp_kernel/` which already contained the C++ API for the C++ kernel. Unfortunately this required removing the existing `third_party/protobuf/rust/cpp_kernel/BUILD` file, because, the Rust parts of the cpp kernel have circular dependencies with the shared code in `third_party/protobuf/rust`.
PiperOrigin-RevId: 886884124