This change moves the `OwnedMessageInterop`, `MessageViewInterop`, and `MessageMutInterop` traits from `codegen_traits.rs` into new `interop.rs` files within both the `cpp_kernel` and `upb_kernel` directories.
The `KernelMessage`, `KernelMessageView`, and `KernelMessageMut` traits in each kernel's `message.rs` are updated to include the respective kernel-specific interop traits. This ensures that the interop functionality is tied to the specific kernel implementation.
The `shared.rs` module now re-exports the interop traits from `crate::__internal::runtime::interop`, which will resolve to the correct kernel's interop module.
It would be possible to take this farther by making the interop traits return strongly typed pointers to `upb_Message*` and `proto2::MessageLite*` instead of `c_void`, but that would be a breaking change.
PiperOrigin-RevId: 892470084
