[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>
