Add basic plugin EP library example in C#

[adrianlizarraga]

• Adds a NuGet package that wraps a basic plugin EP.
• Adds a sample application that uses the plugin EP.

Builds upon #545

[Copilot]

Pull request overview

Adds a C# example for using the “basic plugin” Execution Provider (EP) via a NuGet-wrapped native EP library, along with a sample app that demonstrates registration and inference.

Changes:

• Introduces a NuGet package project (Contoso.ML.OnnxRuntime.EP.Basic) that ships the native EP DLL under runtimes/*/native/ and exposes helper APIs (BasicEp).
• Adds a Windows batch setup script to copy the native DLL into the package layout and build/pack into a local NuGet feed.
• Adds a sample app that consumes the package and runs inference using the plugin EP.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 14 comments.

Show a summary per file

File	Description
plugin_execution_providers/basic/csharp/setup.bat	Builds/packs the NuGet package and stages the native EP DLL into the package runtime folder.
plugin_execution_providers/basic/csharp/readme.md	Documents how to build the native EP, pack the NuGet, and run the sample.
plugin_execution_providers/basic/csharp/nuget.config	Adds a local package source for consuming the newly built nupkg.
plugin_execution_providers/basic/csharp/SampleApp/SampleApp.csproj	Sample app project referencing ONNX Runtime and the EP package/project.
plugin_execution_providers/basic/csharp/SampleApp/Program.cs	Demonstrates registering the EP, selecting an EP device, and running inference.
plugin_execution_providers/basic/csharp/Contoso.ML.OnnxRuntime.EP.Basic/readme.md	Package README content for NuGet.
plugin_execution_providers/basic/csharp/Contoso.ML.OnnxRuntime.EP.Basic/Contoso.ML.OnnxRuntime.EP.Basic.csproj	NuGet package project packing basic_plugin_ep.dll into runtimes/*/native.
plugin_execution_providers/basic/csharp/Contoso.ML.OnnxRuntime.EP.Basic/BasicEp.cs	Helper methods to locate the packaged EP DLL and return EP names.
plugin_execution_providers/basic/csharp/BasicPluginEpExample.sln	Solution containing the package project and sample app.
plugin_execution_providers/basic/csharp/.gitignore	Ignores build artifacts and local feed outputs.

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