onnxruntime

[CoreML EP] Add Pad op support  (#14946)

### Description
<!-- Describe your changes. -->

As title.

- Only support constant mode Pad for CoreML EP for now.
- Enable Pad tests for CoreML with inputs as initializer types.

CoreML Spec for reference:

https://apple.github.io/coremltools/mlmodel/Format/NeuralNetwork.html#paddinglayerparams


### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

Fill operator gaps for ClipChamp models.

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Co-authored-by: rachguo <rachguo@rachguos-Mac-mini.local>
Co-authored-by: rachguo <rachguo@rachguos-Mini.attlocal.net>