Add webgpu plugin EP pipeline

[fs-eire]

Description

This PR introduces a packaging pipeline for ONNX Runtime WebGPU EP plugin for the following platforms:

• win/x64
• win/arm64
• linux/x64
• mac/arm64

Key changes:

CI/CD Pipeline Additions and Improvements:

• Added a new Azure Pipelines YAML pipeline (plugin-webgpu-pipeline.yml) to automate building and packaging the WebGPU plugin for Windows, Linux, and macOS, with parameterized builds for architecture, API version, package version, and build type. The pipeline validates parameter combinations and orchestrates platform-specific packaging stages.
• Introduced modular pipeline stage templates for Linux (plugin-linux-webgpu-stage.yml), macOS (plugin-mac-webgpu-stage.yml), and a top-level packaging stage (plugin-webgpu-packaging-stage.yml) to manage platform-specific build, artifact staging, and publishing processes. [1] [2] [3]

Plugin Versioning and Build Configuration:

• Updated onnxruntime_providers_webgpu.cmake to set the plugin EP version to match ORT_VERSION by default, unless explicitly specified, and to pass this version via a preprocessor definition for consistent version reporting.
• Changed the plugin's reported version in Factory::GetVersionImpl to use the new ORT_PLUGIN_EP_VERSION macro, ensuring the runtime-reported version matches the build configuration.

Codebase Maintenance:

• Added a missing <mutex> include to allocator.h to ensure thread safety and proper compilation.