Initial WebGPU EP checkin (#22318)
### Description
This change introduces the WebGPU EP into ONNX Runtime.
To make the PR as simple as possible, this PR excluded the following:
- C API changes for WebGPU EP
- actual implementation of WebGPU EP. Currently in this PR, WebGPU is a
stub implementation that does not register any kernel.
- Python IO Binding update
- Node.js IO Binding update
This PR now contains only 43 file changes (while the working branch
contains 130+) and hopefully this makes it easier to review.
There is going to be separated PRs for each mentioned above.
Current working branch: #21904
