WebGPU plugin EP Python packaging (#28226)
### Description
<!-- Describe your changes. -->
- Add WebGPU plugin EP Python package build to existing packaging pipeline.
- Add WebGPU plugin EP packaging test pipeline. It contains simple Python package tests to start.
### 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. -->
Put together WebGPU plugin EP Python package.
