[WebNN EP] Add support for Op Pad. (#16732)
### Description
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Support Op Pad for WebNN EP. It aims to support three modes (constant,
reflect and edge). For now, only constant can be tested with Chrome
Canary.
### Motivation and Context
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- If it fixes an open issue, please link to the issue here. -->
Support more models like SD1.5-VAE-encode.
