Enable CoreML by default on macOS wheel builds (#9483)

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

* Include the `executorchcoreml` pybinding in the builds
* Remove separate installation option
* Turn on CoreML by default for macOS builds
* Add a dependency on coremltools for macOS

### Test plan

CI

```
$ rm -rf cmake-out pip-out dist && ./install_executorch.sh
$ ./examples/models/llama/install_requirements.sh
$ .ci/scripts/test_llama.sh -model stories110M -build_tool cmake -dtype fp32 -mode coreml
$ .ci/scripts/test_llama.sh -model stories110M -build_tool cmake -dtype fp32 -mode xnnpack+custom+quantize_kv
```

cc @larryliu0820 @lucylq