[CUDA Plugin EP] Add NuGet packaging pipeline (#28378)
### Description
This PR adds C#/.NET (NuGet) packaging support for the CUDA plugin
Execution Provider, following the pattern established by the WebGPU
plugin EP NuGet packaging (PR #28313). It introduces a new NuGet package
(`Microsoft.ML.OnnxRuntime.EP.Cuda`), the packaging pipeline stage, and
a CI test that validates the produced package on a GPU agent.
### Motivation and Context
Create CUDA plugin EP NuGet package to enable C#/.NET consumers to use
the CUDA plugin EP via NuGet, complementing the existing Python wheel
distribution.
### Key Changes
**C#/.NET Package Infrastructure (`plugin-ep-cuda/csharp/`):**
- `Microsoft.ML.OnnxRuntime.EP.Cuda/` — NuGet package project
(netstandard2.0) with `CudaEp.cs` helper class for native library path
resolution and EP name retrieval
- `pack_nuget.py` — Staging and packing script supporting
`--build-only`/`--pack-only` phases for the CI ESRP signing workflow
- `test/CudaEpNuGetTest/` — Test console app that registers the EP, runs
inference on a Mul model, and validates output
**Pipeline Changes:**
- `plugin-cuda-nuget-packaging-stage.yml` — New NuGet packaging stage
that downloads platform artifacts, builds managed DLL, ESRP-signs it,
packs the NuGet, and signs the .nupkg
- `plugin-cuda-packaging-stage.yml` — Updated to include the NuGet
packaging stage after platform builds
- `plugin-win-cuda-test-stage.yml` — Added `Win_plugin_cuda_nuget_Test`
job that downloads the NuGet package, sets up a local feed, builds and
runs the test project on a GPU agent
**Supported Platforms:**
| RID | Native Binary |
|---|---|
| `win-x64` | `onnxruntime_providers_cuda_plugin.dll` |
| `linux-x64` | `libonnxruntime_providers_cuda_plugin.so` |
| `linux-arm64` | `libonnxruntime_providers_cuda_plugin.so` |
### Related PRs
- #28313 — WebGPU plugin EP NuGet packaging (pattern reference)
- #28375 — C# tests for CUDA Plugin EP (EP name reference:
`CudaPluginExecutionProvider`)
