[WebGPU plugin EP] NuGet packaging (#28313)
### Description
<!-- Describe your changes. -->
This pull request adds C#/.NET (NuGet) packaging support for the WebGPU
plugin Execution Provider, including all necessary project files,
documentation, and helper code. It introduces a new NuGet package
(`Microsoft.ML.OnnxRuntime.EP.WebGpu`), updates the main plugin
documentation to reflect C# support, and provides detailed instructions
and code samples for building, packaging, and using the provider in .NET
applications.
It also has some minor changes for the existing Python packaging setup.
The most important changes are:
**C#/.NET Packaging Infrastructure:**
- Added the `Microsoft.ML.OnnxRuntime.EP.WebGpu` project (`.csproj`) for
NuGet packaging, including metadata, dependency management, and logic to
read the minimum ONNX Runtime version from a shared file. Native
binaries are included per platform, and the README is bundled in the
package.
- Introduced the `WebGpuEp.cs` helper class to resolve the native
library path and EP name at runtime, simplifying registration and usage
in .NET.
**Documentation:**
- Added a detailed `README.md` for the C# package, including usage
instructions, supported platforms, and example code for registering and
using the WebGPU EP in .NET.
- Added a top-level `csharp/README.md` with instructions for building,
packaging, and testing the NuGet package, as well as information on CI
integration and native binary requirements.
### 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. -->
Create WebGPU plugin EP NuGet package.
