Create a CoreML inmemoryfs pybinding target (#9482)

### Summary
Context: https://github.com/pytorch/executorch/pull/9481

We now create the pybinding target for inmemoryfs. Note that the build
recipe is from the wheel builder:


https://github.com/pytorch/executorch/blob/5fdfa511966208c7e237a9e920a7c63f513b4fb7/backends/apple/coreml/runtime/inmemoryfs/setup.py#L17-L38

### Test plan
```
$ cmake -B cmake-out -S . -DEXECUTORCH_BUILD_PYBIND=ON -DEXECUTORCH_BUILD_COREML=ON
$ cmake --build cmake-out -j$(sysctl -n hw.ncpu) --target executorchcoreml

$ ./install_executorch.sh --pybind coreml
```

cc @larryliu0820 @lucylq